U.S. patent number 6,945,610 [Application Number 09/287,707] was granted by the patent office on 2005-09-20 for hydraulic braking system wherein electrically controllable assisting drive force is applied to master cylinder piston upon brake pedal operation.
This patent grant is currently assigned to Toyota Jidosha Kabushiki Kaisha. Invention is credited to Hiroshi Isono, Yasuji Mizutani.
United States Patent |
6,945,610 |
Mizutani , et al. |
September 20, 2005 |
**Please see images for:
( Certificate of Correction ) ** |
Hydraulic braking system wherein electrically controllable
assisting drive force is applied to master cylinder piston upon
brake pedal operation
Abstract
A hydraulically operated braking system including a master
cylinder (12; 300: 500; 600) having a pressurizing piston (34; 322,
324; 504, 506) operatively connected to a brake operating member
(10), to pressurize a fluid in a pressurizing chamber (30, 32; 302,
304; 508, 510), so that a brake cylinder (22-28), is actuated by
the pressurized fluid, and an assisting device (81; 260-272; 109;
538; 612) for applying to the pressurizing piston an assisting
drive force which is other than a primary drive force to be applied
to the pressurizing piston on the basis of a brake operating force
acting on the brake operating member. The assisting device is
electrically controllable to control the assisting drive force.
Inventors: |
Mizutani; Yasuji (Susono,
JP), Isono; Hiroshi (Toyota, JP) |
Assignee: |
Toyota Jidosha Kabushiki Kaisha
(Toyota, JP)
|
Family
ID: |
27310999 |
Appl.
No.: |
09/287,707 |
Filed: |
April 7, 1999 |
Foreign Application Priority Data
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Apr 17, 1998 [JP] |
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10-107517 |
Nov 11, 1998 [JP] |
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10-320247 |
Dec 22, 1998 [JP] |
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10-364575 |
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Current U.S.
Class: |
303/114.1;
303/11; 303/115.5; 303/155 |
Current CPC
Class: |
B60T
8/3275 (20130101); B60T 13/686 (20130101); B60T
8/367 (20130101); B60T 8/441 (20130101); B60T
8/4845 (20130101) |
Current International
Class: |
B60T
13/68 (20060101); B60T 8/48 (20060101); B60T
8/32 (20060101); B60T 8/44 (20060101); B60T
8/36 (20060101); B60T 008/44 () |
Field of
Search: |
;303/10,11,114.1,114.2,155,115.1,115.4,115.5 ;60/545 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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57186571 |
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Nov 1982 |
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JP |
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A-59-149851 |
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Jan 1984 |
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JP |
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U-59-6556 |
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Aug 1984 |
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JP |
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45159 |
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Jan 1992 |
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JP |
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431160 |
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Feb 1992 |
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JP |
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450069 |
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Feb 1992 |
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JP |
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4328064 |
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Nov 1992 |
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JP |
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717388 |
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Jan 1995 |
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JP |
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WO 94 22699 |
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Oct 1994 |
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WO |
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WO 97 18114 |
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May 1997 |
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WO |
|
Primary Examiner: Siconolfi; Robert A.
Attorney, Agent or Firm: Kenyon & Kenyon
Claims
What is claimed is:
1. A hydraulically operated braking system comprising: a brake
operating member operable by an operator; a master cylinder
including a pressurizing piston operatively connected to said brake
operating member and partially defining a pressurizing chamber,
said pressurizing piston being moved by said brake operating member
to pressurize a fluid in said pressurizing chamber; a brake
cylinder actuated by the pressurized fluid received from said
master cylinder; a sensing device for detecting a brake operating
condition quantity indicative of an operating condition of said
brake operating member; and an assisting device for applying to
said pressurizing piston an assisting drive force which is
different than a primary drive force to be applied to said
pressurizing piston on the basis of a brake operating force acting
on said brake operating member, said assisting device applying said
assisting drive force to said pressurizing piston in a first
direction in which said primary drive force is applied to said
pressurizing piston, without application of a force to said brake
operating member in a direction opposite to a second direction in
which said brake operating force acts on said brake operating
member, and wherein said assisting device comprises an assisting
drive force control device electrically operable to control said
assisting drive force on the basis of said brake operating
condition quantity detected by said sensing device, said assisting
drive force control device including changing means for changing a
relationship between said assisting drive force and at least one of
an operating force and an operating stroke of said brake operating
member, detected at least as a part of said brake operating
condition quantity, said relationship being in a normal operation
of the braking system with an operation of said brake operating
member, said changing means including pressure-reducing means for
reducing a pressure of the fluid in said brake cylinder for a given
value of said brake operating force, by reducing said assisting
drive force to be applied to said pressurizing piston in said first
direction, without reducing said primary drive force applied to
said pressurizing piston on the basis of said brake operating
force.
2. A hydraulically operated braking system according to claim 1,
wherein said assisting device further comprises: an assisting
cylinder including an assisting piston operatively connected to
said pressurizing piston, said assisting cylinder having an
assisting pressure chamber which is partially defined by said
assisting piston; a high-pressure source; a reservoir; and a
solenoid-operated pressure control valve device connected to said
high-pressure source, said reservoir and said assisting pressure
chamber, for selectively supplying the fluid from said
high-pressure source to said assisting pressure chamber and
returning the fluid from said assisting pressure chamber to said
reservoir, and wherein said assisting drive force control device
includes a control valve control device for controlling said
solenoid-operated pressure control valve device to control a
pressure of the fluid in said assisting pressure chamber.
3. A hydraulically operated braking system according to claim 1,
wherein said assisting device comprises: an assisting rod
operatively connected to said brake operating member; an
electrically operated actuator for applying an electrically
generated drive force to said assisting rod; and an actuator
control device for controlling said actuator to control said
electrically generated drive force for controlling said assisting
drive force to be applied to said pressurizing piston.
4. A hydraulically operated braking system according to claim 2,
further comprising an emergency closure valve disposed between said
assisting pressure chamber and said solenoid-operated pressure
control valve device and which is normally placed in an open state
for fluid connecting said assisting pressure chamber and said
solenoid-operated pressure control valve device to each other, said
emergency closure valve being brought to a closed state for
disconnecting said assisting pressure chamber and said
solenoid-operated pressure control valve device from each other, in
the event of an abnormality of said solenoid-operated pressure
control valve device.
5. A hydraulically operated braking system according to claim 2,
further comprising an emergency high-pressure source communicating
device for connecting said assisting pressure chamber and said
high-pressure source while by-passing said solenoid-operated
pressure control valve device, in the event of an abnormality of
said solenoid-operated pressure control valve device.
6. A hydraulically operated braking system according to claim 5,
wherein said emergency high-pressure source communicating device
includes a pilot-operated pressure control valve which is connected
to said assisting pressure chamber, said high-pressure source and
said reservoir and which is operated in response to the fluid
pressure in said pressurizing chamber of said master cylinder
received as a pilot pressure, so as to control the fluid pressure
received from said high-pressure source depending upon said pilot
pressure, and apply the controlled fluid pressure to said assisting
pressure chamber.
7. A hydraulically operated braking system according to claim 6,
wherein said pilot-operated pressure control valve is provided in a
by-pass passage which connects said assisting pressure chamber and
said high-pressure source while by-passing said solenoid-operated
pressure control valve device, and said emergency high-pressure
source communicating device further includes a higher-pressure
applying device connected to said by-pass passage, said
solenoid-operated pressure control valve device and said assisting
pressure chamber, said higher-pressure applying device being
operated to apply a higher one of the fluid pressures received from
said solenoid-operated pressure control valve device and said
pilot-operated pressure control valve.
8. A hydraulically operated braking system according to claim 2,
wherein said master cylinder and said assisting cylinder has
respective separate cylinder housings, and said assisting piston is
operatively connected to said pressurizing piston through said
brake operating member, and wherein pressure-receiving surface
areas of said assisting and pressurizing pistons and distances
between a fulcrum of said brake operating member and points of
connection of said assisting and pressurizing pistons to said brake
operating member are determined such that a product of said
pressure-receiving surface area of said assisting piston and said
distance between said fulcrum and said point of connection of said
assisting piston is smaller than a product of said
pressure-receiving surface area of said pressurizing piston and
said distance between said fulcrum and said point of connection of
said pressurizing piston, said braking system further comprising an
emergency fluid communicating device disposed between said
assisting pressure chamber and said pressurizing chamber, said
emergency fluid communicating device being placed in a closed state
disconnecting said assisting pressure chamber and said pressurizing
chamber from each other during an operation of the braking system
when said assisting device is normally operable, and brought to an
open state for fluid communication between said assisting pressure
chamber and said pressurizing chamber in the event of occurrence of
an abnormality of said assisting device during the operation of the
braking system.
9. A hydraulically operating braking system according to claim 2,
wherein said master cylinder and said assisting cylinder are
disposed in series with each other, and said assisting piston has a
pressure-receiving surface area smaller than that of said
pressurizing piston, said braking system further comprising an
emergency communicating device disposed between said assisting
pressure chamber and said pressurizing chamber, said emergency
fluid communicating device being placed in a closed state
disconnecting said assisting pressure chamber and said pressurizing
chamber from each other during an operation of the braking system
when said assisting device is normally operable, and brought to an
open state for fluid communication between said assisting pressure
chamber and said pressuring chamber in the event of occurrence of
an abnormality of said assisting device during the operation of the
braking system.
10. A hydraulically operated braking system according to claim 8,
wherein said emergency fluid communicating device includes a
mechanically operated switch valve which is switched from a closed
state for disconnecting said assisting pressure chamber and said
pressurizing chamber, to an open state for fluid communication
between said assisting pressure chamber and said pressurizing
chamber when the fluid pressure in said high-pressure source is
lowered below a predetermined lower limit.
11. A hydraulically operated braking system according to claim 9,
wherein said emergency fluid communicating device includes a
mechanically operated switch valve which is switched from a closed
state for disconnecting said assisting pressure chamber and said
pressurizing chamber, to an open state for fluid communication
between said assisting pressure chamber and said pressurizing
chamber when the fluid pressure in said high-pressure source is
lowered below a predetermined lower limit.
12. A hydraulically operated braking system according to claim 8,
wherein said emergency fluid communicating device includes an
electrically operated switch valve which is switched from a closed
state for disconnecting said assisting pressure chamber and said
pressurizing chamber, to an open state for fluid communication
between said assisting pressure chamber and said pressurizing
chamber in the event of occurrence of an abnormality of said
assisting device.
13. A hydraulically operated braking system according to claim 9,
wherein said emergency fluid communicating device includes an
electrically operated switch valve which is switched from a closed
state for disconnecting said assisting pressure chamber and said
pressurizing chamber, to an open state for fluid communication
between said assisting pressure chamber and said pressurizing
chamber in the event of occurrence of an abnormality of said
assisting device.
14. A hydraulically operated braking system according to claim 8,
wherein said emergency fluid communicating device is brought to
said open state in the event of occurrence of said abnormality of
said assisting device, if the fluid pressure in said pressurizing
chamber is higher than the fluid pressure in said assisting
pressure chamber by more than a predetermined amount.
15. A hydraulically operated braking system according to claim 9,
wherein said emergency fluid communicating device is brought to
said open state in the event of occurrence of said abnormality of
said assisting device, if the fluid pressure in said pressurizing
chamber is higher than the fluid pressure in said assisting
pressure chamber by more than a predetermined amount.
16. A hydraulically operated braking system according to claim 8,
wherein said emergency fluid communicating device includes (a) a
fluid passage connecting said assisting pressure chamber and said
pressurizing chamber, (b) a switch valve which is disposed in said
fluid passage and which is switched from a closed state
disconnecting said assisting pressure chamber and said pressurizing
chamber, to an open state for communication between said assisting
pressure chamber and said pressurizing chamber, in the event of
said abnormality of said assisting device, and (c) a differential
shut-off valve which is disposed in said fluid passage in series
with said switch valve and which permits a flow of the fluid from
said pressurizing chamber towards said assisting pressure chamber
when the fluid pressure in said pressurizing chamber has become
higher than the fluid pressure in said assisting pressure chamber
by more than said predetermined amount.
17. A hydraulically operated braking system according to claim 9,
wherein said emergency fluid communicating device includes (a) a
fluid passage connecting said assisting pressure chamber and said
pressurizing chamber, (b) a switch which is disposed in said fluid
passage and which is switched from a closed state disconnecting
said assisting pressure chamber and said pressurizing chamber, to
an open state for communication between said assisting pressure
chamber and said pressurizing chamber, in the event of said
abnormality of said assisting device, and (c) a differential
shut-off valve which is disposed in said fluid passage in series
with said switch valve and which permits a flow of the fluid from
said pressurizing chamber towards said assisting pressure chamber
when the fluid pressure in said pressurizing chamber has become
higher than the fluid pressure in said assisting pressure chamber
by more than said predetermined amount.
18. A hydraulically operated braking system according to claim 8,
wherein said emergency fluid communicating device includes an
electrically operated switch valve which is disposed between said
assisting pressure chamber and said pressurizing chamber and which
is switchable between a closed state disconnecting said assisting
pressure chamber and said pressurizing chamber and an open state
for communication between said assisting pressure chamber and said
pressurizing chamber, and a switch valve control means for
switching said electrically operated switch valve from said closed
state to said open state when said assisting device is not normally
operable and when the fluid pressure in said pressurizing chamber
is higher than the fluid pressure in said assisting pressure
chamber by more than said predetermined amount.
19. A hydraulically operated braking system according to claim 9,
wherein said emergency fluid communicating device includes an
electrically operated switch valve which is disposed between said
assisting pressure chamber and said pressurizing chamber and which
is switchable between a closed state disconnecting said assisting
pressure chamber and said pressurizing chamber and an open state
for communication between said assisting pressure chamber and said
pressurizing chamber, and a switch valve control means for
switching said electrically operated switch valve from said closed
state to said open state when said assisting device is not normally
operable and when the fluid pressure in said pressurizing chamber
is higher than the fluid pressure in said assisting pressure
chamber by more than said predetermined amount.
20. A hydraulically operated braking system according to claim 2,
further comprising an emergency reservoir communicating device
disposed between said assisting pressure chamber and said
reservoir, said emergency reservoir communicating device being
placed in a closed state disconnecting said assisting pressure
chamber and said reservoir from each other during an operation of
the braking system when said assisting device is normally operable,
and brought to an open state for fluid communication between said
assisting pressure chamber and said reservoir in the event of
occurrence of an abnormality of said assisting device during the
operation of the braking system.
21. A hydraulically operated braking system according to claim 1,
further comprising; a master reservoir; a fluid passage for fluid
communication between said master reservoir and said pressurizing
chamber of said master cylinder, irrespective of a position of said
pressurizing piston; and a check valve disposed in said fluid
passage, said check valve inhibiting a flow of the fluid from said
pressurizing chamber towards said master reservoir and allowing a
flow of the fluid from said master reservoir towards said
pressurizing chamber.
22. A hydraulically operated braking system according to claim 21,
wherein said master cylinder includes a cylinder housing having a
port connected to said fluid passage and communicating with said
pressurizing chamber, said master cylinder further including a
device for preventing said port from being closed by said
pressurizing piston.
23. A hydraulically operated braking system according to claim 1,
wherein said master cylinder includes (a) a first pressurizing
piston operatively connected to said brake operating member
partially defining a first pressurizing chamber whose volume
increases as said first pressurizing piston is moved, (b) a second
pressurizing piston which said partially defines said first
pressurizing chamber and a second pressurizing chamber in front of
said first pressurizing chamber, so as to separate said first and
second pressurizing chambers from each other, and which is movable
relative to said first pressurizing piston, (c) a second
pressurizing chamber pressurizing device for pressurizing the fluid
in said second pressurizing chamber by supplying a pressurized
fluid from a pressure source external to said master cylinder, into
said second pressurizing chamber, and (d) a volume reduction
preventing device for permitting the volume of said first
pressurizing chamber to be increased as said first pressurizing
piston is advanced from an original position thereof while said
second pressurizing piston is placed in an original position
thereof, and for preventing the volume of the first pressurizing
chamber from being reduced when the fluid pressure in said second
pressurizing chamber is increased by said second pressurizing
chamber pressurizing device while said second pressurizing piston
is placed in said original position.
24. A hydraulically operated braking system according to claim 13,
wherein said original position of said second pressurizing piston
is fully retracted position thereof, and said volume reduction
preventing device is a stopper device for preventing a movement of
said second pressurizing piston from said fully retracted position
in a direction opposite to a direction of an advancing movement of
said second pressurizing piston.
25. A hydraulically operated braking system according to claim 23,
wherein said second pressurizing piston includes a partition
portion for dividing an interior of a cylinder housing of said
master cylinder into said first and second pressurizing chambers,
and a cylindrical portion disposed on one side of said partition
portion which is on the side of said first pressurizing piston,
said original position of said second pressurizing piston being
defined by an abutting contact of a rear open end face of said
cylindrical portion with a rear end face of said cylinder housing,
said stopper device including said rear open end face of said
cylindrical portion and said rear end face of said cylinder
housing, and wherein said first pressurizing piston is slidably
fitted in said cylindrical portion of said second pressurizing
piston.
26. A hydraulically operated braking system according to claim 25,
wherein said first pressuring chamber includes an inner fluid
chamber formed within said cylindrical portion of said second
pressurizing piston and in front of said first pressurizing piston,
and an outer annular fluid chamber formed between an outer
circumferential surface of said second pressurizing piston and an
inner circumferential surface of said cylinder housing, said
cylindrical portion having a communication passage for fluid
communication between said inner fluid chamber and said outer
annular fluid chamber.
27. A hydraulically operated braking system according to claim 26,
wherein said outer annular fluid chamber has a volume which is
reduced as said second pressuring piston is advanced, and said
communication passage functions as a fluid flow restrictor for
restricting a flow of the fluid between said inner fluid chamber
and said outer annular fluid chamber.
28. A hydraulically operated braking system according to claim 23,
wherein said second pressurizing chamber is connected to a wheel
brake cylinder as said brake cylinder for braking a drive wheel of
an automotive vehicle, said braking system further comprising a
solenoid-operated shut-off valve which is disposed between said
second pressurizing chamber pressurizing device and said second
pressurizing chamber and which has an open position for fluid
communication between said second pressurizing chamber pressurizing
device and said second pressurizing chamber, and a closed position
for disconnecting said second pressurizing chamber pressuring
device and said second pressurizing chamber from each other, and a
drive wheel braking pressure control device for controlling the
fluid pressure in said drive wheel brake cylinder while said
solenoid-operated shut-off valve is held in said open state.
29. A hydraulically operated braking system according to claim 1,
further comprising a brake operating force estimating device for
estimating an operating force acting on said brake operating
member, on the basis of the fluid pressure in said pressurizing
chamber and said assisting drive force produced by said assisting
device.
30. A hydraulically operated braking system according to claim 1,
wherein said master cylinder includes a cylinder housing which
cooperates with said pressurizing piston to define said pressuring
chamber, said braking system further comprising a master cylinder
characteristic control device for controlling an amount of the
fluid in said pressurizing chamber of said master cylinder, to
thereby control a relationship between a position of said
pressurizing piston relative to said cylinder housing and the fluid
pressure in said pressurizing chamber.
31. A hydraulically operated braking system according to claim 30,
wherein said master cylinder characteristic control device
comprises: a cylinder housing; a volume-changing piston received in
said cylinder housing of said master cylinder characteristic
control device such that said volume-changing piston is movable
relative to said cylinder housing of said master cylinder
characteristic control device; said volume-changing piston
cooperating with said cylinder housing of said master cylinder
characteristic control device to define a variable-volume chamber
communicating with said pressurizing chamber; and a fluid amount
control device for controlling a relative position of said
volume-changing piston and said cylinder housing of said master
cylinder characteristic control device, to control a volume of said
variable-volume chamber, for thereby controlling the amount of the
fluid in said pressurizing chamber.
32. A hydraulically operated braking system according to claim 31,
wherein said fluid amount control device includes a master cylinder
pressurizing control means for controlling the amount of the fluid
in said pressurizing chamber, on the basis of an operating stroke
of said pressurizing piston and according to a predetermined
rule.
33. A hydraulically operated braking system according to claim 30,
wherein said master cylinder characteristic control device has a
variable-volume chamber connected to a braking fluid chamber in
said brake cylinder and said pressurizing chamber of said master
cylinder, and includes a fluid amount control device for
controlling a volume of said variable-volume chamber to control the
amount of the fluid in said pressurizing chamber, said braking
system further comprising an emergency master cylinder
disconnecting device disposed between said variable-volume chamber
and said pressurizing chamber, said emergency master cylinder
disconnecting device being normally placed in an open state for
fluid communication between said variable-volume chamber and said
pressurizing chamber, and brought to a closed state for
disconnecting said variable-volume chamber and said pressurizing
chamber from each other in the event of an abnormality of said
assisting device.
34. A hydraulically operated braking system comprising: a brake
operating member operable by an operator; a master cylinder
including a cylinder housing and a pressurizing piston operatively
connected to said brake operating member and cooperating with said
cylinder housing to define a pressurizing chamber, said
pressurizing piston being moved by said brake operating member to
pressurize a fluid in said pressurizing chamber; a brake cylinder
actuated by the pressurized fluid received from said master
cylinder; a sensing device for detecting a brake operating
condition quantity indicative of an operating condition of said
brake operating member; and a master cylinder characteristic
control device for controlling an amount of the fluid in said
pressurizing chamber of said master cylinder, on the basis of said
brake operating condition quantity, to thereby change a
relationship between a position of said pressurizing piston
relative to said cylinder housing and the fluid pressure in said
pressurizing chamber in a normal operation of the braking system,
for controlling a fluid pressurizing characteristic of said master
cylinder.
35. A hydraulically operated braking system according to claim 1,
wherein said sensing device is operable to detect, as said brake
operating condition quantity, at least one of a quantity
corresponding to an operating amount of said brake operating member
and a quantity corresponding to a rate of change of said operating
amount, and said assisting drive force control device is operable
to control said assisting drive force on the basis of said at least
one of said quantities corresponding to said operating amount and
said rate of change of said operating amount.
36. A hydraulically operated braking system according to claim 35,
wherein said sensing device includes at least one of a force sensor
for detecting a quantity corresponding to said operating force of
said brake operating member and a stroke sensor for detecting a
quantity corresponding to an operating stroke of said brake
operating member, said assisting drive force control device is
operable to control said assisting drive force on the basis of at
least one of said quantities corresponding to said operating force
and said operating stroke of said brake operating member.
37. A hydraulically operated braking system according to claim 36,
wherein said assisting drive force control device is operable to
control said assisting drive force on the basis of both of said
quantities corresponding to said operating force and said operating
stroke of said brake operating member.
38. A hydraulically operated braking system according to claim 34,
wherein said sensing device includes at least one of a force sensor
for detecting a quantity corresponding to an operating force of
said brake operating member and a quantity corresponding to a
stroke sensor for detecting an operating stroke of said brake
operating member, and said master cylinder characteristic control
device is operable to control the amount of the fluid in said
pressurizing chamber of said master cylinder on the basis of at
least one of said quantities corresponding to said operating force
and said operating stroke of said brake operating member.
39. A hydraulically operated braking system according to claim 38,
wherein said master cylinder characteristic control device is
operable to control the amount of the fluid in said pressurizing
chamber of said master cylinder on the basis of both of said
quantities corresponding to said operating force and said operating
stroke of said brake operating member.
40. A hydraulically operated braking system according to claim 1,
wherein said sensing device further detects a vehicle running
condition quantity indicative of a running condition of an
automotive vehicle having a wheel which is braked by said brake
cylinder, and said assisting drive force control device further
includes means for changing a relationship between said assisting
drive force and said brake operating condition quantity on the
basis of said vehicle running condition quantity during said normal
operation of the braking system.
41. A hydraulically operated braking system comprising: a brake
operating member operable by an operator; a master cylinder
including a pressuring piston operatively connected to said brake
operating member and partially defining a pressurizing chamber,
said pressurizing piston being moved by said brake operating member
to pressurize a fluid in said pressurizing chamber; a brake
cylinder actuated by the pressurized fluid received from said
master cylinder; a sensing device for detecting a brake operating
condition quantity indicative of an operating condition of said
brake operating member; and an assisting device for applying to
said pressurizing piston an assisting drive force which is
different than a primary drive force to be applied to said
pressurizing piston on the basis of a brake operating force acting
on said brake operating member, said assisting device including
only one actuator operable to generate said assisting drive force,
and wherein said assisting device comprises an assisting drive
force control device electrically operable to control said only one
actuator for controlling said assisting drive force on the basis of
said brake operating condition quantity detected by said sensing
device, said assisting drive force control device including
changing means for changing a relationship between said assisting
drive force and at least one of an operating force and an operating
stroke of said brake operating member detected at least as a part
of said brake operating condition quantity, said relationship being
in a normal operation of the braking system with an operation of
said brake operating member, said changing means including
pressure-reducing means for reducing a pressure of the fluid in
said brake cylinder for a given value of said brake operating
force, by reducing a force of operation of said only one actuator
to reduce said assisting drive force, without reducing said primary
drive force applied to said pressurizing piston on the basis of
said brake operating force.
42. A hydraulically operated braking system comprising: a brake
operating member operable by an operator; a master cylinder
including a pressurizing piston operatively connected to said brake
operating member and partially defining a pressurizing chamber,
said pressurizing piston being moved by said brake operating member
to pressurize a fluid ins aid pressurizing chamber; a brake
cylinder actuated by the pressurized fluid received from said
master cylinder; a sensing device for detecting a brake operating
condition quantity indicative of an operating condition of said
brake operating member; and an assisting device for applying to
said pressurizing piston an assisting drive force which is
different than a primary drive force to be applied to said
pressurizing piston on the basis of a brake operating force acting
on said brake operating member, such that said assisting drive
force is applied to said pressurizing piston in a first direction
in which said primary drive force is applied to said pressurizing
piston, said assisting device not including an actuator operable to
generate a force to be applied to said brake operating member in a
direction opposite to a second direction in which said brake
operating force acts on said brake operating member, and wherein
said assisting device comprises an assisting drive force control
device electrically operable to control said assisting drive force
on the basis of said brake operating condition quantity detected by
said sensing device, said assisting drive force control device
including changing means for changing a relationship between said
assisting drive force and at least one of an operating force and an
operating stroke of said brake operating member detected at least
as a part of said brake operating condition quantity, said
relationship being in a normal operation of the braking system with
an operation of said brake operating member.
Description
This application is based on Japanese Patent Applications Nos.
10-107517, 10-320247 and 10-364575 filed Apr. 17, Nov. 11 and Dec.
22, 1998, respectively, the contents of which are incorporated
hereinto by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates in general to a hydraulically
operated braking system, and more particularly to a hydraulically
operated braking system including an assisting device for boosting
a drive force to be applied to a pressurizing piston of a master
cylinder.
2. Discussion of the Related Art
An example of a hydraulically operated braking system including
such an assisting device as described above is disclosed in
JP-A-4-328064. This braking system includes (1) a master cylinder
having a pressurizing piston operatively connected to a brake
operating member to pressurize a working fluid in a pressurizing
chamber, (2) a brake cylinder for actuating a brake device based on
the pressure of the pressurized fluid, and (3) an assisting device
for applying to the pressurizing piston an assisting drive force
which is different than a primary drive force to be applied to the
pressurizing piston on the basis of a brake operating force acting
on the brake operating member. In this braking system wherein the
primary drive force and the assisting drive force are both applied
to the pressurizing piston, the fluid pressure in the pressurizing
chamber can be boosted, permitting the brake device to produce an
increased braking force for a given value of the brake operating
force. However, the assisting drive force to be generated by the
assisting device is simply proportional to the brake operating
force. That is, the assisting device disclosed in the
above-identified publication is not capable of producing the
assisting drive force which is not proportional to the brake
operating force.
SUMMARY OF THE INVENTION
It is therefore an object of this invention to provide a
hydraulically operated braking system comprising an assisting
device capable of producing an assisting drive force in a
non-proportional relationship with the brake operating force.
The above object may be achieved according to any one of the
following modes of the present invention, each of which is numbered
like the appended claims and depends from the other mode or modes,
where appropriate, so as to indicate various technical features and
possible combinations of elements in preferred forms of the
invention. However, it is to be understood that the present
invention is not limited to those specific modes, features or
combinations which will be described.
(1) A hydraulically operated braking system comprising: a brake
operating member operable by an operator; a master cylinder
including a pressurizing piston operatively connected to the brake
operating member and partially defining a pressurizing chamber, the
pressurizing piston being moved by the brake operating member to
pressurize a fluid in the pressurizing chamber; a brake cylinder
actuated by the pressurized fluid received from the master
cylinder; and an assisting device for applying to the pressurizing
piston an assisting drive force which is other than a primary drive
force to be applied to the pressurizing piston on the basis of a
brake operating force acting on the brake operating member, the
assisting device being electrically controllable to control the
assisting drive force.
In the hydraulically operated braking system of the present
invention constructed as described above, the assisting drive force
to be applied to the pressurizing piston of the master cylinder is
electrically controllable, so that the assisting drive force can be
controlled in a non-proportional relationship with the brake
operating force acting on the brake operating member. Further, the
electric control of the assisting drive force results in improved
ease and freedom of control the assisting drive force.
The assisting drive force produced by the assisting device may be
applied to the pressurizing piston, either simultaneously and
together with the primary drive force produced upon operation of
the brake operating member, or alone and without simultaneous
application of the primary drive force. In the latter case, the
assisting drive force is applied to the pressurizing piston when
the brake operating member is placed in the non-operated position
(without the brake operating force acting thereon). In this case,
the hydraulically operated braking system is automatically actuated
with the assisting drive force applied to the pressurizing piston,
namely, actuated to effect automatic brake application without an
operation of the brake operating member by the operator.
(2) A hydraulically operated braking system according to the above
mode (1), wherein the assisting device includes an assisting drive
force control device electrically operable to control the assisting
drive force on the basis of at least one of a brake operating
condition quantity indicative of an operating condition of the
brake operating member and a vehicle running condition quantity
indicative of a running condition of an automotive vehicle having a
wheel which is braked by the brake cylinder.
In the hydraulically operated braking system according to the above
mode (2), the assisting drive force is electrically controlled on
the basis of at least one of the brake operating condition quantity
and the vehicle running condition quantity. For instance, the brake
operating condition quantity may be an operating amount (operating
force or stroke) of the brake operating member, a rate of change of
the operating amount (rate of change of the operating force or
stroke, or a combination of the operating amount and the rate of
change of the operating amount. For example, the assisting drive
force may be controlled as a quadratic function of the operating
amount of the brake operating member. In this case, the rate of
change of a braking force (produced by the brake cylinder) with the
operating amount can be made higher when the operating amount is
relatively large, than when the operating amount is relatively
small, so that the braking sensitivity is comparatively high when
the operating amount is relatively large.
The vehicle running condition quantity may be a running speed of
the vehicle, an acceleration value of the vehicle, a slipping
condition quantity of the vehicle wheel, a turning condition
quantity of the vehicle, or a running environment quantity of the
vehicle. The slipping condition of the vehicle wheel includes
slipping of the wheel while the wheel is braked, and slipping of
the wheel while the wheel is driven. The slipping condition
quantity of the wheel may be a slip ratio of the wheel, a slip
amount or speed of the wheel, a rate of change of the slip ratio or
amount, or a combination of the slip ratio or amount and the rate
of change thereof. The turning condition quantity may be a yaw rate
of the vehicle, a difference between rotating speeds of right and
left wheels of the vehicle, a rate of change of the yaw rate or the
wheel speed difference, or a combination of the yaw rate or wheel
speed difference and the rate of change thereof. The running
environment quantity indicates the environment in which the vehicle
is running.
For instance, the assisting drive force may be controlled so as to
be larger when the vehicle running speed upon initiation of a
braking operation of the braking system is comparatively high than
when the vehicle running speed is comparatively low. In this
instance, the required stopping distance of the vehicle can be
reduced. If the assisting drive force is controlled to be smaller
when the slipping condition quantity of the wheel being braked is
comparatively large than when the slipping condition quantity is
comparatively small. In this case, the braking stability of the
vehicle can be improved.
The vehicle running environment quantity may be a friction
coefficient of the road surface, or an ambient temperature. Since
the running condition of the vehicle can be estimated from the
vehicle running environment quantity, this vehicle running
environment quantity may be considered to be the vehicle running
condition quantity. For instance, it is possible to consider that
the slipping condition quantity of the wheel tends to be larger
when the friction coefficient of the road surface is comparatively
low than when the friction coefficient is comparatively high. When
the ambient temperature is comparatively low and the viscosity of
the working fluid is comparatively high, it is possible to consider
that the braking effect provided by the braking system tends to be
delayed. Accordingly, the assisting drive force may be controlled
to be larger to increase the master cylinder pressure to a higher
level when the ambient temperature is relatively low, so that the
delay of the braking effect at the relatively low ambient
temperature can be reduced. The vehicle running environment
quantity may be a quantity relating to a distance between the
vehicle and a person or any other object in front of the vehicle.
When this distance (which indicates a risk of collision of the
vehicle with the object) is relatively small, or when a rate of
reduction of this distance is relatively high (a rate of approach
of the vehicle to the object) is relatively high, it is possible to
consider that there is a relatively high risk of the vehicle
collision with the object. To rapidly stop the vehicle, therefore,
it is desirable to control the assisting drive force to be larger
when the distance is relatively small or when the rate of reduction
of the distance is relatively high.
A control mode in which the assisting drive force is controlled on
the basis of the wheel slipping condition quantity, vehicle turning
condition quantity or vehicle collision risk may be considered to
be one form of an anti-lock control, a traction control, a vehicle
turning stability control, or an emergency brake control of the
braking system. In the anti-lock control, for instance, the braking
pressure applied to the brake cylinder may be controlled so as to
hold the slip amount or ratio of the wheel within an optimum range,
by changing the assisting drive force while maintaining the primary
drive force at a constant value. Where the assisting device is
operable to produce the assisting drive force without an operation
of the brake operating member, the traction control or vehicle
turning stability control can be effected by controlling the
assisting drive force. The emergency brake control may be effected
for the purpose of increasing the braking force during operation of
the brake operating member, or for the purpose of generating a
braking force without or prior to an operation of the brake
operating member. Where the assisting device is operable without an
operation of the brake operating member, the emergency brake
control may be effected for the latter purpose.
The braking system may include a running condition control device
in addition to the assisting device, so that the anti-lock control,
traction control and vehicle turning stability control is effected
by the running condition control device. In this case, the
assisting drive force produced by the assisting device may be
controlled depending upon whether the running condition control
device is in operation. For example, the assisting drive force may
be controlled to be comparatively small while the running condition
control device is in operation, so that the assisting drive force
has a reduced degree of influence on the anti-lock, traction or
vehicle turning stability control. In this case, a control quantity
(one or zero, for example) indicating whether the running condition
control device is in operation may be considered to be the vehicle
running condition quantity.
(3) A hydraulically operated braking system according to the above
mode (1) or (2), wherein the assisting device comprises: an
assisting cylinder including an assisting piston operatively
connected to the pressurizing piston, the assisting cylinder having
an assisting pressure chamber which is partially defined by the
assisting piston; a high-pressure source; a reservoir; a
solenoid-operated pressure control valve device connected to the
high-pressure source, the reservoir and the assisting pressure
chamber, for selectively supplying the fluid from the high-pressure
source to the assisting pressure chamber and returning the fluid
from the assisting pressure chamber to the reservoir; and a control
valve control device for controlling the solenoid-operated pressure
control valve device to control a pressure of the fluid in the
assisting pressure chamber.
In the above mode of the braking system, a force acting on the
assisting piston based on the fluid pressure in the assisting
pressure chamber is increased to increase the assisting drive force
to be applied to the pressurizing piston, as the fluid pressure in
the assisting pressure chamber is increased.
The assisting cylinder may be disposed in parallel or in series
with the master cylinder. Where the assisting cylinder is disposed
in parallel with the master cylinder, piston rods of the
pressurizing piston and the assisting piston are operatively
connected, at their ends remote from the pistons, to the brake
operating member, such that those ends are spaced from each other
in the longitudinal direction of the brake operating member. Where
the assisting cylinder is disposed in series with the master
cylinder, a portion of the piston rod of the pressurizing piston
may be utilized as the piston rod of the assisting piston.
Alternatively, the pressurizing and assisting pistons may be formed
integrally with each other. In this latter case, the assisting
cylinder and the master cylinder desirably employ the same cylinder
housing in which the pressurizing chamber and the assisting
pressure chamber are formed. On the other hand, the
solenoid-operated pressure control valve device may consist of a
single solenoid-operated pressure control valve, a plurality of
solenoid-operated shut-off valve, or at least one solenoid-operated
directional control valve. Where the solenoid-operated pressure
control valve device includes a pressure increase control valve
disposed between the high-pressure source and the assisting
pressure chamber, and a pressure reduction control valve disposed
between the reservoir and the assisting pressure chamber, a
solenoid-operated shut-off valve may be provided between the
pressure reduction control valve and the assisting pressure
chamber, and/or between the pressure reduction control valve and
the reservoir. In this case, the solenoid-operated shut-off valve
is controlled such that the shut-off valve is open while the
pressure reduction control valve is in a pressure reducing state
for permitting the pressurized fluid to be discharged from the
assisting pressure chamber into the reservoir, and is closed while
the pressure reduction control valve is in a pressure holding state
for inhibiting a discharge flow of the fluid from the assisting
pressure chamber. This shut-off valve prevents the pressurized
fluid from being discharged from the assisting pressure chamber
towards the reservoir even when the fluid leaks from the pressure
reduction control valve. Thus, the shut-off valve is controlled in
response to the operating state of the pressure reduction control
valve.
(4) A hydraulically operated braking system according to the above
mode (1) or (2), wherein the assisting device comprises: an
assisting rod operatively connected to the brake operating member;
an electrically operated actuator for applying an electrically
generated drive force to the assisting rod; and an actuator control
device for controlling the actuator to control the electrically
generated drive force for controlling the assisting drive force to
be applied to the pressurizing piston.
The electrically generated drive force to be applied to the
assisting rod is controlled by controlling the electrically
operated actuator, so that the assisting drive force to be applied
to the pressurizing piston is controlled. The actuator may be an
electric motor or a piezoelectric element or elements.
(5) A hydraulically operated braking system according, to the above
mode (3), further comprising an emergency closure valve disposed
between the assisting pressure chamber and the solenoid-operated
pressure control valve device and which is normally placed in an
open state for fluid connecting the assisting pressure chamber and
the solenoid-operated pressure control valve device to each other,
the emergency closure valve being brought to a closed state for
disconnecting the assisting pressure chamber and the
solenoid-operated pressure control valve device from each other, in
the event of an abnormality of the solenoid-operated pressure
control valve device.
The braking systems according to the above mode (5) and any one of
the following modes of this invention are improvements of the
braking system according to the above mode (1); (2) or (3).
The "abnormality of the solenoid-operated pressure control valve
device" described above with respect to the above mode (5) includes
a failure of the solenoid-operated pressure control valve itself to
normally function, and an electrical abnormality that prevents the
application of an electric current to the solenoid-operated
pressure control valve device. Where the pressure control valve
device includes the pressure increase control valve between the
assisting pressure chamber and the high-pressure source, and a
pressure reduction control valve between the assisting pressure
chamber and the reservoir, the pressure increase or pressure
control valve cannot be closed due to sticking due to a foreign
matter contained in the working fluid. In this case, the
pressurized fluid is continuously fed from the high-pressure source
to the assisting pressure chamber, or is continuously discharged
from the assisting pressure chamber towards the reservoir.
Accordingly, the fluid pressure in the assisting pressure chamber
may be increased to an abnormally high level or lowered to an
abnormally low level. Such an abnormal variation of the fluid
pressure in the assisting pressure chamber can be avoided by the
emergency closure shut-off valve which is placed in the closed
state to disconnect the assisting pressure chamber and the
solenoid-operated pressure control valve device from each other, in
the even of occurrence of an abnormality of the pressure increase
or reduction control valve. The abnormality that the pressure
increase control valve cannot be closed may be detected if the
fluid pressure in the assisting pressure chamber is increased even
when the pressure increase control valve is commanded to be closed,
or if the actual value of the fluid pressure in the assisting
pressure chamber is higher than a desired or target value by more
than a predetermined amount and if the absolute value of the
difference between the actual and desired values is increasing. An
abnormality that the abnormality of the pressure reduction control
valve cannot be closed can be detected if the fluid pressure in the
assisting pressure chamber is reduced even when the pressure
reduction control valve is commanded to be closed, or if the actual
value of the fluid pressure in the assisting pressure chamber is
lower than a desired or target value by more than a predetermined
amount and if the absolute value of the difference of the actual
and desired values is increasing.
The solenoid-operated pressure control valve device may be arranged
to be brought to a pressure holding state for inhibiting a flow of
the fluid from the assisting pressure chamber towards the reservoir
a flow of the fluid from the high-pressure source into the
assisting pressure chamber, in the event of an electrical
abnormality that prevents an electric current from being applied to
the pressure control valve device. In this case, the fluid may leak
from the pressure control valve device placed in the pressure
holding state. The provision of the emergency closure valve for
disconnecting the assisting pressure chamber and the
solenoid-operated pressure control valve device is effective to
prevent or reduce a variation in the fluid pressure in the
assisting pressure chamber in the event of such an electrical
abnormality.
(6) A hydraulically operated braking system according to any one of
the above modes (3), (4) and (5), further comprising an emergency
high-pressure source communicating device for connecting the
assisting pressure chamber and the high-pressure source while
by-passing the solenoid-operated pressure control valve device, in
the event of an abnormality of the solenoid-operated pressure
control valve device.
The "abnormality of the solenoid-operated pressure control valve
device" described above with respect to the above mode (6) may be
an abnormality that prevents the pressurized fluid from being
supplied from the high-pressure source to the assisting pressure
chamber. Where the solenoid-operated pressure control valve device
includes a pressure increase control valve disposed between the
assisting pressure chamber and the high-pressure source, the
abnormality of the pressure control valve device may be an
abnormality that the pressure increase control valve cannot be
opened, making it impossible to supply the pressurized fluid to the
assisting pressure chamber through the pressure increase control
valve. In this case, the high-pressure source can be communicated
with the assisting pressure chamber through the emergency
high-pressure source communicating device, so that the pressurized
fluid can be supplied from the high-pressure source to the
assisting pressure chamber. The pressure increase control valve
cannot be opened, due to sticking of a movable member of the
control valve caused by a foreign matter contained in the fluid or
rusting of the movable member, or alternatively due to an
electrical defect of the control valve. In either of these
abnormalities, the emergency high-pressure source communicating is
effective only where the pressurized fluid having a high pressure
is available from the high-pressure source.
The emergency high-pressure source communicating device may include
a by-pass passage which connects the high-pressure source and the
assisting pressure chamber while by-passing the solenoid-operated
pressure control valve device, and a normally-open solenoid
operated shut-off valve provided in the by-pass passage. The
shut-off valve is opened when an electric current is not applied to
its solenoid coil, so that the pressurized fluid can be supplied
from the high-pressure source to the assisting pressure chamber
even when the pressure increase control valve cannot be opened due
to the electrical abnormality.
Where the breaking system includes both of the features of the
above modes (5) and (6), the emergency high-pressure source
communicating device is preferably arranged to connect the
assisting pressure chamber and the high-pressure source to each
other even when the emergency closure valve cannot be switch from
the closed state to the open state.
(7) A hydraulically operated braking system according to the above
mode (6), wherein the emergency high-pressure source communicating
device includes a pilot-operating pressure control valve which is
connected to the assisting pressure chamber, the high-pressure
source and the reservoir and which is operated in response to the
fluid pressure in the pressurizing chamber of the master cylinder
received as a pilot pressure, so as to control the fluid pressure
received from said high-pressure source depending upon the pilot
pressure, and apply the controlled fluid pressure to the assisting
pressure chamber.
The use of the pilot-operated pressure control valve permits the
non-electrical control of the fluid pressure of the high-pressure
source depending upon the master cylinder pressure, so that the
controlled fluid pressure is applied to the assisting pressure
chamber. Therefore, even when the solenoid-operated pressure
control valve device fails to normally operate, for example, the
assisting cylinder can be actuated to control the assisting drive
force depending upon the master cylinder pressure, as long as the
high-pressure source is normal. Where the high-pressure source
includes a pump and an accumulator, the assisting cylinder can be
actuated even after the pump becomes inoperable, as long as the
pressurized fluid is stored in the accumulator.
(8) A hydraulically operated braking system according to the above
mode (7), wherein the pilot-operated pressure control valve is
provided in a by-pass passage which connects the assisting pressure
chamber and the high-pressure source while by-passing the
solenoid-operated pressure control valve device, and the emergency
high-pressure source communicating device further includes a
higher-pressure applying device connected to the by-pass passage,
the solenoid-operated pressure control valve device and the
assisting pressure chamber, the higher-pressure applying device
being operated to apply a higher one of the fluid pressures
received from the solenoid-operated pressure control valve device
and the pilot-operated pressure control valve.
In the braking system according to the above mode (8), the higher
one of the fluid pressures which the higher-pressure applying
device receive from the pilot-operated pressure control valve and
the solenoid-operated pressure control valve device is applied to
the assisting pressure chamber. Accordingly, the assisting pressure
chamber is actuated with the consistently higher fluid pressure,
than in the case where a predetermined of the above-indicated two
fluid pressures is applied to the assisting pressure chamber. A
normally-closed solenoid-operated shut-off valve may be disposed
between the pilot-operated pressure control valve and the
high-pressure source, so that this shut-off valve is opened in the
event of occurrence of an abnormality of the solenoid-operated
pressure control valve device. In this case, the pilot-operated
pressure control valve is operable only when the solenoid-operated
pressure control valve device is not normally operable.
(9) A hydraulically operated braking system according to any one of
the above mode (3) and (5)-(8), wherein the master cylinder and the
assisting cylinder has respective separate cylinder housings, and
the assisting piston is operatively connected to the pressurizing
piston trough the brake operating member, and wherein
pressure-receiving surface areas of the assisting and pressurizing
pistons and distances between a fulcrum of the brake operating
member and points of connection of the assisting and pressurizing
pistons to the brake operating member are determined such that a
product of the pressure-receiving surface area of the assisting
piston and the distance between the fulcrum and the point of
connection of the assisting piston is smaller than a product of the
pressure-receiving surface area of the pressurizing piston and the
distance between the fulcrum and the point of connection of the
pressurizing piston, the braking system further comprising an
emergency fluid communicating device disposed between the assisting
pressure chamber and the pressurizing chamber, the emergency fluid
communicating device being placed in a closed state disconnecting
the assisting pressure chamber and the pressurizing chamber from
each other during an operation of the braking system when the
assisting device is normally operable, and brought to an open
position for fluid communication between the assisting pressure
chamber and the pressurizing chamber in the event of occurrence of
an abnormality of the assisting device during the operation of the
braking system.
The "abnormality of the assisting device" described above with
respect to the above mode (9) may be an abnormality that prevents
the application of a pressurized fluid to the assisting pressure
chamber, an abnormality that prevents the control of the fluid
pressure in the assisting pressure chamber, or an abnormality that
prevents flows of the fluid into and from the assisting pressure
chamber. These abnormalities may arise from an electrical defect of
the assisting device, or an abnormality associated with the
high-pressure source, the solenoid-operated pressure control valve
device or the control valve control device.
In the braking system according to the above mode (9), the
pressurizing chamber of the master cylinder and the assisting
pressure chamber of the assisting cylinder are communicated with
each other through the emergency fluid communicating device, in the
event of an abnormality of the assisting device. Accordingly, the
fluid flows between the assisting and pressurizing chambers are
permitted, although the fluid flows between the assisting pressure
chamber and the high-pressure source or the reservoir are not
possible. When the brake operating member is operated, the fluid is
supplied from the pressurizing chamber of the master cylinder to
the assisting pressure chamber, so that the assisting piston is
permitted to be moved. This arrangement prevents the assisting
cylinder from preventing an operation of the brake operating
member. When the brake operating member is released, the fluid is
returned from the assisting pressure chamber to the pressurizing
chamber.
Further, the product of the pressure-receiving surface area of the
assisting piston and the distance between the fulcrum of the brake
operating member and the point of connection of the assisting
piston to the brake operating member is made smaller than the
product of the pressure-receiving surface area of the pressurizing
piston and the distance between the fulcrum of the brake operating
member and the point of connection of the pressurizing piston to
the brake operating member. Accordingly, the fluid pressurized in
the pressurizing chamber can be supplied to the brake cylinder,
although the pressurized fluid is supplied from the pressurizing
chamber also to the assisting pressure chamber, as explained below.
Further, the supply of the pressurizing fluid from the pressurizing
chamber to the assisting pressure chamber causes the assisting
cylinder to provide an assisting drive force (which is larger than
zero), which is applied to the assisting piston through the brake
operating member. Thus, the present arrangement is substantially
equivalent to a reduction of the pressure-receiving surface area of
the pressurizing piston, and permits boosting of the fluid pressure
generated in the pressurizing piston for a given operating force
acting on the brake operating member.
As described in detail in the DETAILED DESCRIPTION OF THE PREFERRED
EMBODIMENTS, the fluid pressure in the assisting pressure chamber
of the assisting cylinder becomes equal to that in the pressurizing
chamber of the master cylinder when the assisting pressure chamber
and the pressurizing chamber are communicated with each other.
Where the master cylinder and the assisting cylinder are disposed
in parallel with each other, as shown in FIG. 2, the fluid pressure
P.sub.M ' in the pressurizing chamber (master cylinder pressure
P.sub.M ') is expressed by the following equation:
In the above equation (1), S.sub.S, S.sub.M, L.sub.M, F, L.sub.F
represent the following: S.sub.S : pressure-receiving surface area
of the assisting piston, L.sub.S : distance between the fulcrum of
the brake operating member and the point of connection of the
assisting piston to the brake operating member, S.sub.M :
pressure-receiving surface area of the pressurizing piston, L.sub.M
: distance between the fulcrum of the brake operating member and
the point of connection of the pressurizing piston to the brake
operating member, F: operating force applied to the brake operating
member by the operator, and L.sub.F : Distance between the fulcrum
and the point at which the operating force acts on the brake
operating member.
Since S.sub.S.times.L.sub.S is smaller than S.sub.M.times.L.sub.M,
as described above, the master cylinder pressure P.sub.M ' will not
be a negative pressure, so that the fluid is prevented from being
discharged from the brake cylinder into the pressurizing
chamber.
On the other hand, the master cylinder pressure P.sub.M when the
assisting drive force is zero is expressed by the following
equation (2):
By using the above equation (2), the above equation (1) can be
converted into the following equation (3):
It will be understood from the above equation (3) that a ratio of
the master cylinder pressure P.sub.M ' when the assisting, pressure
chamber and the pressurizing chamber are communicated with each
other to the master cylinder pressure P.sub.M when the assisting
drive force is zero is expressed by the following equation (4):
Since S.sub.S.times.L.sub.S <S.sub.M.times.L.sub.M, the ratio
P.sub.M '/P.sub.M is larger than 1. Thus, the braking force can be
made larger when the assisting pressure chamber and the
pressurizing chamber are communicated with each other than when
these two chambers are not communicated with each other. As the
brake operating member is operated, the fluid pressurized in the
pressurizing chamber is supplied to the assisting pressure chamber,
resulting in an increase in the fluid pressure in the assisting
pressure chamber, and a force based on the fluid pressure in the
assisting pressure chamber acts on the assisting piston, so that
the assisting drive force based on the force acting on the
assisting piston is applied to the pressurizing piston through the
brake operating member.
(10) A hydraulically operated braking system according to any one
of the above modes (3) and (5)-(8), wherein the master cylinder and
the assisting cylinder are disposed in series with each other, and
the assisting piston has a pressure-receiving surface area smaller
than that of the pressurizing piston, the braking system further
comprising an emergency communicating device disposed between the
assisting pressure chamber and the pressurizing chamber, the
emergency fluid communicating device being placed in a closed state
disconnecting the assisting pressure chamber and the pressurizing
chamber from each other during an operation of the braking system
when the assisting device is normally operable, and brought to an
open position for fluid communication between the assisting
pressure chamber and the pressuring chamber in the event of
occurrence of an abnormality of the assisting device during the
operation of the braking system.
The braking system according to the above mode (10) is
substantially identical with the braking system according to the
above mode (9) as modified such that the distance L.sub.M is made
equal to the distance L.sub.S. In both of these modes (9) and (10),
the assisting cylinder is arranged so that the assisting piston
produces a moment that acts on the brake operating member in the
direction in which the brake operating force acts on the brake
operating member.
(11) A hydraulically operated braking system according to the above
mode (9) or (10), wherein the emergency fluid communicating device
includes a mechanically operated switch valve which is switched
from a closed state for disconnecting the assisting pressure
chamber and the pressurizing chamber, to an open state for fluid
communication between the assisting pressure chamber and the
pressurizing chamber when the fluid pressure in the high-pressure
source is lowered below a predetermined lower limit.
When the fluid pressure in the high-pressure source is lowered
below the predetermined lower limit due to an abnormality of the
high-pressure source, the assisting pressure chamber is generated
disconnected from both the high-pressure source and the reservoir,
by the solenoid-operated pressure control valve device. In this
case, the fluid flows into and from the assisting pressure chamber
are not possible. However, the mechanically operated switch vale
placed in the open state permits the fluid communication between
the assisting pressure chamber and the pressurizing chamber,
namely, permits the fluid flows between these two chambers. The
mechanically operated switch valve is more reliable than a
solenoid-operated switch valve, and is typically a pilot-operated
switch valve which receives the fluid pressure of the high-pressure
source as a pilot pressure.
(12) A hydraulically operated braking system according to the above
mode (9) or (10), wherein the emergency fluid communicating device
includes an electrically operated switch valve which is switched
from a closed state for disconnecting the assisting pressure
chamber and the pressurizing chamber, to an open state for fluid
communication between the assisting pressure chamber and the
pressurizing chamber in the even of occurrence of an abnormality of
the assisting device.
(13) A hydraulically operated braking system according to any one
of the above modes (9)-(12), wherein the emergency fluid
communicating device is brought to the open state in the event of
occurrence of the abnormality of the assisting device, if the fluid
pressure in the pressurizing chamber is higher than the fluid
pressure in the assisting pressure chamber by more than a
predetermined amount.
In the braking system according to the above mode (13), the
pressurizing chamber of the master cylinder and the assisting
pressure chamber of the assisting cylinder are communicated with
each other when the fluid pressure in the pressurizing chamber is
higher than the fluid pressure the assisting pressure chamber by
more than the predetermined amount, due to the abnormality of the
assisting device. The communication between the pressurizing
chamber and the assisting pressure chamber with each other through
the emergency fluid communicating device has substantially the same
effect as a reduction in the inside diameter of the master
cylinder. Accordingly, the operating stroke of the pressurizing
piston of the master cylinder is increased. However, the
communication is not effected immediately after the assisting
device has become abnormal, but is effected only after the fluid
pressure in the pressurizing chamber has become higher than that in
the assisting pressure chamber by more than the predetermined
amount. This arrangement results in a reduction of the operating
stroke of the pressurizing piston. For instance, the predetermined
amount may be determined that the emergency fluid communicating
device is held closed disconnecting the pressurizing chamber and
the assisting pressure chamber until the brake cylinder has been
filled with the fluid and started to provide a braking effect. This
arrangement is effective to reduce the required operating stroke of
the pressurizing piston and therefore the required operating stroke
of the brake operating member, with substantially no deterioration
of the function of the assisting device.
(14) A hydraulically operated braking system according to any one
of the above modes (9)-(13), wherein the emergency fluid
communicating device includes (a) a fluid passage connecting the
assisting pressure chamber and the pressurizing chamber, (b) a
switch valve which is disposed in the fluid passage and which is
switched from a closed state disconnecting the assisting pressure
chamber and the pressurizing chamber, to an open state for
communication between the assisting pressure chamber and the
pressurizing chamber, in the event of the abnormality of the
assisting device, and (c) a differential shut-off valve which is
disposed in the fluid passage in series with the switch valve and
which permits a flow of the fluid from the pressurizing chamber
towards the assisting pressure chamber when the fluid pressure in
the pressurizing chamber has become higher than the fluid pressure
in the assisting pressure chamber by more than the predetermined
amount.
In the braking system according to the above mode (13), a flow of
the fluid from the pressurizing chamber towards the assisting
pressure chamber is inhibited even after the switch valve is
switched to the open state, as long as the fluid pressure in the
pressurizing pressure chamber is not higher than the fluid pressure
in the assisting pressure chamber by more than the predetermined
amount, that is, as long as the differential shut-off valve is held
closed. In this respect, the differential shut-off valve may be
considered to be a flow restrictor device, or a device for limiting
the flow of the fluid from the pressurizing chamber into the
assisting pressure chamber.
The predetermined amount indicated above, that is, the opening
pressure difference of the differential shut-off valve may be a
fixed value determined by a biasing force of a spring incorporated
in the differential shut-off valve, or may be variable depending
upon an electric energy applied to a coil incorporated in the
valve. Where the opening pressure difference is variable, the
relationship between the operating stroke of the pressurizing
piston and the fluid pressure of the pressurizing chamber can be
controlled.
A check valve which inhibits a fluid flow from the pressurizing
chamber towards the assisting pressure chamber and allows a fluid
flow in the opposite direction may be disposed in parallel with the
differential shut-off valve. The check valve permits the fluid to
be returned from the assisting pressure chamber to the pressurizing
chamber when the brake operating member is released.
(15) A hydraulically operated braking system according to the above
modes (9), (10), (12) and (13), wherein the emergency fluid
communicating device includes an electrically operated switch valve
which is disposed between the assisting pressure chamber and the
pressurizing chamber and which is switchable between a closed state
disconnecting the assisting pressure chamber and the pressurizing
chamber and an open state for communication between the assisting
pressure chamber and the pressurizing chamber, and a switch valve
control means for switching the electrically operated switch valve
from the closed state to the open state when the assisting device
is not normally operable and when the fluid pressure in the
pressurizing chamber is higher than the fluid pressure in the
assisting pressure chamber by more than the predetermined
amount.
The braking system according to the above mode (15) also permits
the fluid in the pressurizing chamber to be sufficiently
pressurized while reducing the operating stroke of the pressurizing
piston, in the event of an abnormality of the assisting device.
The electrically operated switch valve indicated above may be
replaced by a mechanically operated switch valve which is switched
from the closed state to the open state when the fluid pressure in
the pressurizing chamber has become higher than the fluid pressure
in the assisting pressure chamber by more than the predetermined
amount while the fluid pressure in the high-pressure source is
lower than a predetermined lower limit. For instance, the
mechanically operated switch valve may be adapted to be opened when
a force based on the pressure difference of the pressurizing
chamber and the assisting pressure chamber has become larger than a
force based on the fluid pressure in the high-pressure source.
(16) A hydraulically operated braking system according to any one
of the above modes (3), (5)-(8) and (13)-(15), further comprising
an emergency reservoir communicating device disposed between the
assisting pressure chamber and the reservoir, the emergency
reservoir communicating device being placed in a closed state
disconnecting the assisting pressure chamber and the reservoir from
each other during an operation of the braking system when the
assisting device is normally operable, and brought to an open state
for fluid communication between the assisting pressure chamber and
the reservoir in the event of occurrence of an abnormality of the
assisting device during the operation of the braking system.
In the braking system according to the above mode (16), the
assisting pressure chamber and the reservoir are communicated with
each other through the emergency reservoir communicating device to
permit the fluid flows between the assisting pressure chamber and
the reservoir, in the event of an abnormality of the assisting
device, that is, when the fluid flows between the assisting
pressure chamber into and from the high-pressure source or the
reservoir are impossible. When the brake operating member is
operated, the fluid is supplied from the reservoir into the
assisting pressure chamber. When the brake operating member is
released, the fluid is returned from the assisting pressure chamber
back to the reservoir. The reservoir with which the assisting
pressure chamber is communicated with the emergency reservoir
communicating device may be a master reservoir used for the master
cylinder, or may be a reservoir separate from the master reservoir.
The assisting cylinder is usually disposed near the master cylinder
(or may be formed integrally with the master cylinder). In this
respect, the emergency reservoir communicating device is desirably
disposed in a fluid passage connecting the assisting pressure
chamber and the master reservoir, so that the fluid passage may be
shortened.
Where the emergency reservoir communicating device is provided in
the braking system according to any one of the above modes
(13)-(15), this device permits the fluid flows between the
assisting pressure chamber and the reservoir, even if the fluid
flows between the assisting pressure chamber and the pressurizing
chamber are restricted by the emergency fluid communicating device,
differential shut-off valve or electrically operated switch valve
indicated above, in the event of an abnormality of the assisting
device.
(17) A hydraulically operated braking system according to any one
of the above modes (1)-(16), further comprising; a master
reservoir; a fluid passage for fluid communication between the
master reservoir and the pressurizing chamber of the master
cylinder, irrespective of a position of the pressurizing piston;
and a check valve disposed in the fluid passage, the check valve
inhibiting a flow of the fluid from the pressurizing chamber
towards the master reservoir and allowing a flow of the fluid from
the master reservoir towards the pressurizing chamber.
In the conventional master cylinder, the pressurizing chamber is
connected and disconnected to and from the master reservoir,
depending upon the position of the pressurizing piston. For
instance, the conventional master cylinder has a port which is
formed in its cylinder housing and which communicates with the
master reservoir through a fluid passage, and includes a cup seal
provided on the pressurizing piston. In this conventional master
cylinder, the port is open for fluid communication of the
pressurizing chamber with the master reservoir when the
pressurizing piston is placed in the original or fully retracted
position. When the pressurizing piston is advanced from the
original position, the port is closed by the cup seal, and the
pressurizing chamber is disconnected from the master reservoir, so
that the fluid pressure in the pressurizing chamber is increased as
the pressurizing piston is advanced. As the volume of the
pressurizing chamber increases with a retracting movement of the
pressurizing piston towards the original position, the fluid is
permitted to flow from the master reservoir into the pressurizing
chamber, to thereby prevent the fluid pressure in the pressurizing
chamber from being lowered below the atmospheric level. When the
pressurizing piston has been returned to its original position, the
port is opened to the pressurizing chamber, for communicating the
pressurizing chamber with the master reservoir.
Another type of conventional master cylinder has an inlet check
valve disposed between the cylinder housing and the pressurizing
piston, or between pressurizing pistons. In this type of master
cylinder, the inlet check valve is operated from the open state to
the closed state when the pressurizing piston is advanced. In the
closed state of the inlet check valve, the pressurizing chamber is
disconnected from the master reservoir, so that the fluid pressure
in the pressurizing chamber can be increased as the pressurizing
piston is advanced. When the pressurizing piston is retracted, the
inlet check valve is brought to the open state, the fluid is
supplied from the master reservoir to the pressurizing chamber, to
prevent the fluid pressure in the pressurizing chamber from being
lowered below the atmospheric level. When the pressurizing piston
has been returned to its original position, the inlet check valve
is restored to its open state for fluid communication of the
pressurizing chamber with the master reservoir.
In the master cylinder of the braking system according to the above
mode (17), the pressurizing chamber and the master reservoir are
held in communication with each other through the fluid passage,
irrespective of the position of the pressurizing piston. Namely,
the pressurizing chamber is not disconnected from the master
reservoir depending upon the position of the pressurizing piston,
but the pressurizing chamber is always held in communication with
the master reservoir through the fluid passage. However, the check
valve is provided in this fluid passage, so as to permit the flow
of the fluid from the master reservoir towards the pressurizing
chamber, and inhibit the fluid flow in the opposite direction.
Since the fluid is prevented by the check valve from being
discharged from the pressurizing chamber into the master reservoir,
the fluid pressure in the pressurizing chamber can be increased as
the pressurizing piston is advanced. Further, since the fluid is
permitted to be fed from the master reservoir into the pressurizing
chamber, the fluid pressure in the pressurizing chamber is
prevented from being lowered below the atmospheric level when the
pressurizing piston is retracted to its original or fully retracted
position. This arrangement eliminates an increase in the operating
stroke of the pressurizing piston, which is required in the
conventional master cylinder, to selectively open and close the cup
seal or the inlet check valve. Accordingly, the required
longitudinal or axial dimension of the master cylinder can be
reduced in the present braking system. Where the master cylinder is
provided in series with the assisting cylinder, the required
overall length of the master cylinder and the assisting cylinder is
relatively large. Therefore, the feature of the above mode (17) is
particularly advantageous when it is provided in combination of the
series arrangement of the master cylinder and the assisting
cylinder.
The assisting device in the braking system according to the above
mode (17) may utilize the master reservoir, or employs an exclusive
reservoir different from the master reservoir.
The feature of the above mode (17) is available independently of
the feature of any one of the above modes (1)-(16) of the present
invention.
(18) A hydraulically operated braking system according to the above
mode (17), wherein the master cylinder includes a cylinder housing
having a port connected to the fluid passage and communicating with
the pressurizing chamber, the master cylinder further including a
device for preventing the port from being closed by the
pressurizing piston.
In the braking system according to the above mode (18), the port is
held in communication with the pressurizing chamber, so that the
pressurizing chamber is held in communication with the master
reservoir. For instance, the device for preventing the port from
being closed by the pressurizing piston includes annular radial
walls which are formed on the inner circumferential surface of the
cylinder housing and with which the pressurizing piston
fluid-tightly and slidably engages. In this case, the port will not
be closed by the pressurizing piston, irrespective of the position
of the pressurizing piston relative to the cylinder housing.
(19) A hydraulically operated braking system according to any one
of the above modes (1)--(19), wherein the master cylinder includes
(a) a first pressurizing piston operatively connected to the brake
operating member partially defining a first pressurizing chamber
whose volume decreases as the first pressurizing piston is moved,
(b) a second pressurizing piston which the partially defines the
first pressurizing chamber and a second pressurizing chamber in
front of the first pressurizing chamber, so as to separate the
first and second pressurizing chambers from each other, and which
is movable relative to the first pressurizing piston, (c) a second
pressurizing chamber pressurizing device for pressurizing the fluid
in the second pressurizing chamber by supplying a pressurized fluid
from a pressure source external to the master cylinder, into the
second pressurizing chamber, and (d) a volume reduction preventing
device for permitting the volume of the first pressurizing chamber
to be increased as the first pressurizing piston is advanced from
an original position thereof while said second pressurizing piston
is placed in an original position thereof, and for preventing the
volume of the first pressurizing chamber from being reduced when
the fluid pressure in the second pressurizing chamber is increased
by the second pressurizing chamber pressurizing device while said
second pressurizing piston is placed in said original position.
In the master cylinder of the braking system according to the above
mode (19), the first and second pressurizing chambers of the master
cylinder are separated from each other by the second pressurizing
piston. As the brake operating member is operated, the first
pressurizing piston is advanced, and the volume of the first
pressurizing chamber is reduced so as to increase the fluid
pressure in the first pressurizing chamber. As a result, the second
pressurizing piston is advanced so as to increase the fluid
pressure in the second pressurizing chamber. The volume reduction
preventing device prevents reduction of the volume of the first
pressurizing chamber due to an increase in the fluid pressure in
the second pressurizing chamber with an advancing movement of the
first pressurizing piston from the original position while the
first pressurizing piston is placed in the original position.
Therefore, the fluid pressure in the second pressurizing chamber
can be increased by the second pressurizing chamber pressurizing
device by supplying the pressurized fluid from the external
pressure source to the second pressurizing chamber, without
increasing the fluid pressure in the first pressurizing chamber.
Thus, the fluid pressure in the brake cylinder communicating with
the second pressurizing chamber can be increased without increasing
the fluid pressure in the brake cylinder communicating with the
first pressurizing chamber. When the brake operating member is
operated in this condition, the first pressurizing piston is
permitted to be advanced by the operated brake operating member, so
that the fluid pressure in the first pressurizing chamber is
increased, to increase the fluid pressure in the brake cylinder
communicating with the first pressurizing chamber. When the brake
operating member is operated while the fluid pressure in the second
pressurizing chamber is relatively high, a sufficient amount of the
pressurized fluid can be supplied from the first pressurizing
chamber to the corresponding brake cylinder, so that this brake
cylinder can be actuated without a trouble.
The second pressurizing chamber pressurizing device may be a device
exclusively used for pressurizing the second pressurizing chamber
independently of the assisting device. Alternatively, the assisting
device may be utilized as the second pressurizing chamber
pressurizing device.
The feature of the above mode (19) may be available independently
of the feature of any one of the above modes (1)-(18).
(20) A hydraulically operated braking system according to the above
mode (19), wherein the original position of the second pressurizing
piston is a fully retracted position thereof, and the volume
reduction preventing device is a stopper device for preventing a
movement of the second pressurizing piston from the fully retracted
position in a direction opposite to a direction of an advancing
movement of the second pressurizing piston.
When the second pressurizing piston is placed in the fully
retracted position, the second pressurizing piston is not moved
from the fully retracted position in the retracting direction even
when the fluid pressure in the second pressurizing chamber is
increased. Accordingly, an increase in the fluid pressure in the
second pressurizing chamber will not cause the volume of the first
pressurizing chamber to be reduced. The stopper device may be
provided at an intermediate portion or rear end portion of the
master cylinder.
(21) A hydraulically operated braking system according to the above
mode (19) or (20), wherein the second pressurizing piston includes
a partition portion for dividing an interior of a cylinder housing
of the master cylinder into the first and second pressurizing
chambers, and a cylindrical portion disposed on one side of the
partition portion which is on the side of the first pressurizing
piston, the original position of the second pressurizing piston
being defined by an abutting contact of a rear open end face of the
cylindrical portion with a rear end face of the cylinder housing,
the stopper device including said rear open end face of said
cylindrical portion and said rear end face of said cylinder
housing, and wherein the first pressurizing piston is slidably
fitted in the cylindrical portion of the second pressurizing
piston.
The fully retracted position of the second pressurizing piston is
defined by the abutting contact of the rear open end face of the
cylindrical portion of the second pressurizing piston and the rear
end face of the cylinder housing of the master cylinder.
While the second pressurizing piston is movable relative to the
cylinder housing of the master cylinder, it is not desirable that
the relative movement of the second pressurizing piston and the
cylinder housing is effected such that the outer circumferential
surface of the cylindrical portion is in contact with the inner
circumferential surface of the cylinder housing. In this respect,
it is desirable to form an annular radial wall on one of the outer
circumferential surface of the cylindrical portion and the inner
circumferential surface of the cylinder housing, so that the other
of those outer and inner circumferential surfaces fluid-tightly and
slidably engages the annular radial wall. The annular radial wall
may be formed on both of those outer and inner circumferential
surfaces so that the annular radial walls formed on these
circumferential surfaces engage the circumferential surfaces.
The partition portion of the second pressurizing piston indicated
above may take the form of a cylinder or a circular disc. The
partition portion may be formed either integrally with or
separately from the cylindrical portion which has the rear open end
face indicated above. For instance, the second pressurizing piston
may consist of two integrally formed cylindrical portions one of
which has the rear open end face indicated above and the other of
which has a bottom wall serving as the partition portion.
(22) A hydraulically operated braking system according to the above
mode (21), wherein the first pressuring chamber includes an inner
fluid chamber formed within the cylindrical portion of the second
pressurizing piston and in front of the first pressurizing piston,
and an outer annular fluid chamber formed between an outer
circumferential surface of the second pressurizing piston and an
inner circumferential surface of the cylinder housing, the
cylindrical portion having a communication passage for fluid
communication between the inner fluid chamber and the outer annular
fluid chamber.
In the braking system according to the above mode (22), the fluid
is supplied from the first pressurizing chamber into the outer
annular fluid chamber, and then into the brake cylinder. The outer
annular fluid chamber may be a variable-volume fluid chamber whose
volume is reduced as the second pressurizing chamber is advanced,
or a constant-volume fluid chamber whose volume is held constant.
However, the fluid pressure in the outer annular fluid chamber can
be made higher when this fluid chamber is a variable-volume fluid
chamber.
The variable-volume outer annular fluid chamber may be defined by
the inner circumferential surface of the cylinder housing, the
outer circumferential surface of the second pressurizing piston, a
first annular radial wall formed on the inner circumferential
surface of the cylinder housing, and a second annular radial wall
formed on the outer circumferential surface of the second
pressurizing piston. The first annular radial wall is formed in
front of the second annular radial wall. The partition wall
fluid-tightly and slidably engages the first annular-radial wall,
while the cylindrical portion fluid-tightly and slidably engages
the second annular radial wall. In this arrangement, the volume of
the variable-volume outer annular fluid chamber is reduced as the
second pressurizing piston is advanced. Where the partition portion
takes the form of a cylinder, the maximum operating stroke of the
second pressurizing piston can be made comparatively large, so that
the amount of change of the volume of the variable-volume outer
annular fluid chamber can be made relatively large, and the weight
of the partition portion can be reduced.
(23) A hydraulically operated braking system according to the above
mode (22), wherein the outer annular fluid chamber has a volume
which is reduced as the second pressuring piston is advanced, and
the communication passage functions as a fluid flow restrictor for
restricting a flow of the fluid between the inner fluid chamber and
the outer annular fluid chamber.
In the braking system according to the above mode (23), there may
arise a difference between the fluid pressures in the inner fluid
chamber and the outer annular fluid chamber, in the presence of the
fluid flow restrictor therebetween. When the brake operating member
is operated at a relatively high speed so as to rapidly reduce the
volume of the inner fluid chamber, the fluid flow from the inner
fluid chamber into the variable-volume outer annular fluid chamber
is restricted by the fluid flow restrictor, so that the fluid
pressure in the inner fluid chamber is increased, thereby causing a
fluid pressure difference between the outer and inner fluid
chambers. On the basis of this fluid pressure difference, the
second pressurizing piston having a larger pressure-receiving
surface area is advanced, and the volume of the variable-volume
outer annular fluid chamber is reduced. As a result, the fluid
pressure in the brake cylinder can be increased at a higher rate
when the second pressuring piston is advanced, than when the first
pressurizing piston whose pressure-receiving surface area is
smaller than that of the second pressurizing piston is advanced
relative to the second pressurizing piston.
(24) A hydraulically operated braking system according to any one
of the above modes (19)-(23), wherein the second pressurizing
chamber is connected to a wheel brake cylinder as the brake
cylinder for braking a drive wheel of an automotive vehicle, the
braking system further comprising a solenoid-operated shut-off
valve which is disposed between the second pressurizing chamber
pressurizing device and the second pressurizing chamber and which
has an open position for fluid communication between the second
pressurizing chamber pressurizing device and the second
pressurizing chamber, and a closed position for disconnecting the
second pressurizing chamber pressuring device and the second
pressurizing chamber from each other, and a drive wheel braking
pressure control device for controlling the fluid pressure in the
drive wheel brake cylinder while the solenoid-operated shut-off
valve is held in said open state.
In the braking system according to the above mode (24), the fluid
can be supplied from the second pressurizing chamber pressurizing
device to the second pressurizing chamber through the
solenoid-operated shut-off valve held in the open state, so that
the fluid pressure in the drive wheel brake cylinder can be
increased. The pressure of the fluid delivered from the second
pressurizing chamber pressurizing device may be controllable or may
be held constant. Where the output pressure of the second
pressurizing chamber pressurizing device is held constant, it is
preferable to provide a fluid pressure control valve device between
the second pressurizing chamber and the drive wheel brake cylinder,
for controlling the fluid pressure to be applied to the drive wheel
brake cylinder.
The drive wheel braking pressure control device may include at
least one of a drive wheel traction control device and a vehicle
running or turning stability control device. In a drive wheel
traction control effected by the drive wheel traction control
device, the pressurizing fluid can be supplied to the drive wheel
brake cylinder with the solenoid-operated shut-off valve held in
the open state, without an operation of the brake operating member,
and the fluid pressure in the drive wheel brake cylinder can be
controlled to optimize the slipping state of the drive wheel being
driven. In this condition, the first pressurizing piston can be
advanced, so that the fluid pressure in the first pressurizing
chamber can be rapidly increased by an operation of the brake
operating member. Thus, the vehicle can be braked with a high
response to the operation of the brake operating member, even while
the drive wheel traction control is effected. A vehicle turning
stability control for braking the wheel brake cylinder so as to
control the yaw moment of the vehicle can be similarly effected
under the control of the vehicle turning stability control
device.
The assisting device described above with respect to the above mode
(3) can be utilized as the second pressurizing chamber pressurizing
device, and a solenoid-operated shut-off valve which may be
included in the emergency fluid communicating device described with
respect to the above mode (9), (10), (12) or (14) may be utilized
as the solenoid-operated shut-off valve in the braking system of
the above mode (24). The solenoid-operated shut-off valve may be
considered to be included in the second pressurizing chamber
pressurizing device or in the drive wheel braking pressure control
device.
(25) A hydraulically operated braking system according to any one
of the above modes (1)-(24), further comprising a brake operating
force estimating device for estimating an operating force acting on
the brake operating member, on the basis of the fluid pressure in
the pressurizing chamber and the assisting drive force produced by
the assisting device.
In the hydraulically operated braking system according to the above
mode (25), the operating force acting on the brake operating member
is estimated by the brake operating force estimating device, on the
basis of the fluid pressure in the pressurizing chamber of the
master cylinder and the assisting drive force produced by the
assisting device. Therefore, the instant braking system eliminates
a brake operating force detecting device for detecting the
operating force of the brake operating member, so that the cost of
manufacture of the present braking system can be reduced. To
control the assisting device, it is required to use a master
cylinder pressure detecting device for detecting the fluid pressure
in the pressurizing chamber, and an assisting drive force detecting
device for detecting the assisting drive force produced by the
assisting device. Where the assisting drive force corresponds to
the fluid pressure in the assisting pressure chamber described
above with respect to the above mode (3), the assisting drive force
detecting device may be adapted to detect the fluid pressure in the
assisting pressure chamber. These master cylinder pressure
detecting device and the assisting drive force detecting device may
be utilized to estimate the operating force of the brake operating
member, so that the operating force detecting device may be
eliminated. Even where the operating force detecting device is
used, it is not required to be an expensive one that is capable of
accurately detecting the brake operating force over a sufficiently
wide range. That is, the brake operating force estimating device
may be used together with a relatively inexpensive brake operating
force detecting device. In this case, the output of the brake
operating force detecting device is used where the output is
sufficiently accurate, and the output of the brake operating force
estimating device is used where the output of the brake operating
force detecting device is not sufficiently accurate. In this case,
too, the braking system is available at a relatively low cost.
As described in detail in the DETAILED DESCRIPTION OF THE PREFERRED
EMBODIMENTS, the brake operating force can be estimated as
described below, by way of example. Where the master cylinder and
the assisting cylinder are disposed in parallel with each other, as
illustrated in FIG. 2, the operating force F can be estimated
according to the following equation:
where, F.sub.S : assisting drive force, F.sub.M : force (which may
be considered a braking force) based on the fluid pressure in the
pressurizing chamber of the master cylinder, L.sub.S : distance
between the fulcrum of the brake operating member and a point at
which a force based on the fluid pressure in the assisting pressure
chamber acts on the brake operating member, L.sub.M : distance
between the fulcrum and a point at which the force based on the
fluid pressure in the pressurizing chamber acts on the brake
operating member, L.sub.F : distance between the fulcrum and a
point at which the brake operating force F acts on the brake
pedal.
The force F.sub.M is equal to a product of the fluid pressure
P.sub.M in the pressurizing chamber of the master cylinder and a
pressure-receiving surface area S.sub.M of the pressurizing piston.
That is, F.sub.M =P.sub.M.multidot.S.sub.M. The assisting drive
force F.sub.S is a product of the fluid pressure P.sub.S in the
assisting pressure chamber and a pressure-receiving surface area
S.sub.S of the assisting piston. That is, F.sub.S
=P.sub.S.multidot.S.sub.S.
Where the master cylinder and the assisting cylinder are disposed
in series with each other, as illustrated in FIG. 16, the distances
L.sub.S and L.sub.M are equal to each other, and the brake
operating force F can be estimated according to the following
equation:
The pressure-receiving surface area S.sub.S of the assisting piston
is equal to (S.sub.M -S.sub.0 which is smaller than the
pressure-receiving surface area S.sub.M of the pressurizing
piston.
Where the brake operating force detecting device is adapted to
detect, as the brake operating force F, a reaction force F' applied
from the pressurizing piston to the brake operating member, the
reaction force F' and the brake operating force F have a
relationship represented by the following equation:
Therefore, the reaction force F' can be estimated according to the
following equation:
The feature of the above mode (25) is available independently of
the feature of any one of the above modes (1)-(24).
(26) A hydraulically operated braking system according to any one
of the above modes (1)-(25), wherein the master cylinder includes a
cylinder housing which cooperates with the pressurizing piston to
define the pressuring chamber, the braking system further
comprising a master cylinder characteristic control device for
controlling an amount of the fluid in the pressurizing chamber of
the master cylinder, to thereby control a relationship between a
position of the pressurizing piston relative to the cylinder
housing and the fluid pressure in the pressurizing chamber.
In the braking system according to the above mode (26), the amount
of the fluid in the pressurizing chamber of the master cylinder can
be changed by the master cylinder characteristic control device, so
that the relative position of the pressurizing piston and the
cylinder housing can be changed, whereby the relationship between
this relative position and the fluid pressure in the pressurizing
chamber can be changed by the master cylinder characteristic
control device. That is, the fluid pressurizing characteristic of
the master cylinder can be controlled by the master cylinder
characteristic control device. When the brake operating member is
operated, the pressurizing piston is advanced so as to reduce the
volume of the pressurizing chamber. When the fluid is supplied from
the master cylinder characteristic control device to the
pressurizing chamber, upon operation of the brake operating member,
the operating stroke of the pressurizing piston is reduced by an
amount corresponding to the amount of the fluid supplied to the
pressurizing chamber. When the fluid is discharged from the
pressurizing chamber into the master cylinder characteristic
control device, the operating stroke of the pressurizing piston is
increased by an amount corresponding to the amount of the fluid
discharged from the pressurizing chamber. Namely, the operating
stroke of the pressurizing piston decreases with an increase in the
amount of the fluid supplied to the pressurizing chamber, and
increases with a decrease in the amount of the fluid discharged
from the pressurizing chamber. Thus, the operating stroke of the
pressurizing piston can be controlled by controlling the amount of
the fluid in the pressurizing chamber, so that the relationship
between the operating stroke and the fluid pressure in the
pressuring chamber (hereinafter referred to as "master cylinder
pressure") can be controlled by the master cylinder characteristic
control device.
The master cylinder characteristic control device may be adapted to
control the amount of the fluid in the pressurizing piston,
depending upon the operating stroke of the pressurizing piston, or
irrespective of the operating stroke. The amount of the fluid to be
supplied to or discharged from the pressurizing chamber may be
controlled such that the master cylinder pressure linearly
increases with an increase in the operating stroke of the
pressurizing piston. Alternatively, a predetermined amount of the
fluid may be rapidly supplied from the master cylinder
characteristic control device to the pressurizing chamber during an
initial idling stroke of the brake operating member following the
initiation of an operation of the brake operating member detected
by a suitable switch. In the former case, the master cylinder
pressure is controlled to correspond to the operating stroke of the
brake operating member, that is, the amount of operation of the
brake operating member by the operator. In the latter case, the
predetermined amount of the fluid to be suppled to the pressurizing
chamber may be equal to the amount of the fluid required for the
so-called "fast filling" of the master cylinder, or a portion of
the "fast filling" amount. In this case, the required operating
stroke of the brake operating member can be reduced. Further, a
predetermined constant amount of the fluid may be supplied to the
pressurizing chamber for a unit amount of increase of the operating
stroke of the pressurizing piston. The required operating stroke
can be reduced with an increase in this constant amount of the
fluid. When the fluid is discharged from the pressurizing chamber
into the master cylinder characteristic control device, the
required operating stroke increases with an increase in the amount
of the fluid discharged from the pressurizing chamber.
As described above, the master cylinder characteristic control
device permits the operating stroke of the pressurizing piston to
be controlled with respect to the master cylinder pressure. In this
respect, the master cylinder characteristic control device may be
considered to be a device for controlling the operating stroke of
the pressurizing piston. The control of the operating stroke will
become more apparent by the following explanation.
The master cylinder pressure increases with an operating force
acting on the brake operating member as the pressurizing piston is
advanced with an operation of the brake operating member by the
operating force. If a certain amount of the fluid is supplied from
the master cylinder characteristic control device to the
pressurizing chamber while the operating force is held constant,
the pressurizing piston is retracted, by a distance corresponding
to the amount of the fluid supplied to the pressurizing chamber, so
that the required operating stroke of the pressurizing piston is
reduced. Conversely, if a certain amount of the fluid is discharged
from the pressurizing chamber into the master cylinder
characteristic control device, the required operating stroke is
increased. Thus, the master cylinder characteristic control device
is capable of controlling the required operating stroke of the
pressurizing piston.
The feature of the above mode (26) is available independently of
the feature of any one of the features of the above modes
(1)-(25).
(27) A hydraulically operated braking system according to the above
mode (26), wherein the master cylinder characteristic control
device comprises: a cylinder housing; a volume-changing piston
received in the cylinder housing of the master cylinder
characteristic control device such that the volume-changing piston
is movable relative to the cylinder housing of the master cylinder
characteristic control device, the volume-changing piston
cooperating with the cylinder housing of the master cylinder
characteristic control device to define a variable-volume chamber
communicating with the pressurizing chamber; and a fluid amount
control device for controlling a relative position of the
volume-changing piston and the cylinder housing of the master
cylinder characteristic control device, to control a volume of the
variable-volume chamber, for thereby controlling the amount of the
fluid in the pressurizing chamber.
In the braking system according to the above mode (27) wherein the
pressurizing chamber and the variable-volume chamber communicate
with each other, the amount of the fluid in the pressurizing
chamber can be controlled by controlling the volume of the
variable-volume chamber. The fluid is supplied into the
pressurizing chamber when the volume of the variable-volume chamber
is reduced, and the fluid is discharged from the pressurizing
chamber when the volume of the variable-volume chamber is
increased. Thus, by controlling the amount of change of the volume
of the variable-volume chamber, the amount of the fluid to be
supplied to the pressurizing chamber and the amount of the fluid to
be discharged from the pressurizing chamber can be controlled.
The volume of the variable-volume chamber is changed by moving the
volume-changing piston. The volume-changing piston may be moved by
a fluid pressure or an electric actuator such as an electric motor
and a piezoelectric device.
Where the volume-changing piston is moved by a fluid pressure, the
cylinder housing of the master cylinder characteristic control
device cooperates with the volume-changing piston to define a
volume control chamber on one side of the volume-changing piston
which is remote from the variable-volume chamber. The
volume-changing piston is moved to a position of equilibrium
between the fluid pressure in the volume control chamber and the
fluid pressure in the variable-volume chamber. In this case, the
fluid amount control device includes a high-pressure source, a
reservoir, a solenoid-operated pressure control valve device
connected to the high-pressure source, reservoir and volume control
chamber, and a control valve control device for controlling the
solenoid-operated pressure control valve device to control the
fluid pressure in the volume control chamber. The cylinder housing
and the volume control piston which define the variable-volume
chamber and the volume control chamber are considered to constitute
a master cylinder characteristic control cylinder for controlling
the pressurizing characteristic of the master cylinder. This master
cylinder characteristic control cylinder may be considered to be a
stroke adjusting cylinder for adjusting the operating stroke of the
pressurizing piston and the operating stroke of the brake operating
member. The stroke adjusting cylinder may be used to reduce the
required operating stroke of the pressurizing piston.
Where the volume control piston is moved by a drive force produced
by an electric actuator. The position of the volume control piston
and the velocity of movement of this piston can be controlled by
controlling the electric actuator. The fluid amount control device
may include such an electric actuator, a motion converting or
transmitting mechanism for transmitting a motion or displacement of
the electric actuator to the volume-changing piston, and an
actuator control device for controlling the electric actuator.
(28) A hydraulically operated braking system according to the above
mode (27), wherein the fluid amount control device includes a
master cylinder pressurizing control means for controlling the
amount of the fluid in the pressurizing chamber, on the basis of an
operating stroke of the pressurizing piston and according to a
predetermined rule.
In the braking system according to the above mode (28), the fluid
in the pressurizing chamber is controlled to control the fluid
pressurizing of the master cylinder, on the basis of the operating
stroke of the pressurizing piston and according to the
predetermined rule. The relationship between the operating stroke
of the pressurizing piston and the amount of the fluid in the
pressurizing chamber can be changed by changing the rule. For
instance, the predetermined rule may be a predetermined
relationship between the operating stroke S of the pressurizing
piston and the fluid pressure P.sub.M (master cylinder pressure) in
the pressurizing chamber of the master cylinder. An example of this
relationship is indicated by solid line in FIG. 4, which
relationship is represented by an equation P.sub.M
=k.multidot.S.sup.2. The relationship may be represented by an
equation P.sub.M =k(S-a).sup.2 +b.
If the assisting drive force is controlled by the assisting device
force control device as described above with respect to the above
mode (2) of this invention, a relationship between the power of the
brake operating force and the master cylinder pressure P.sub.M may
be controlled as indicated in the graph of FIG. 5, and a
relationship between the operating stiffness of the brake operating
member and the master cylinder pressure P.sub.M may be controlled
as indicated in the graph of FIG. 6. Each of these relationships
may be considered to be an example of the fluid pressurizing
characteristic of the master cylinder, and may also be considered
to be a characteristic of the braking effect to be provided by the
present braking system. The power is a ratio of an amount or change
of the master cylinder pressure P.sub.M to an amount of change of a
product of the operating stroke S and force F of the brake
operating member. That is, the power is represented by [dP.sub.M
/(S.multidot.dF=F.multidot.dS)]. On the other hand, the operating
stiffness is a ratio of an amount of change of the operating force
F to an amount of change of the operating stroke S, that is,
dF/ds.
(29) A hydraulically operated braking system according to any one
of the above modes (26)-(28), wherein the master cylinder
characteristic control device has a variable-volume chamber
connected to a braking fluid chamber in the brake cylinder and the
pressurizing chamber of the master cylinder, and includes a fluid
amount control device for controlling a volume of the
variable-volume chamber to control the amount of the fluid in the
pressurizing chamber, the braking system further comprising an
emergency master cylinder disconnecting device disposed between the
variable-volume chamber and the pressurizing chamber, the emergency
master cylinder disconnecting device being normally placed in an
open state for fluid communication between the variable-volume
chamber and the pressurizing chamber, and brought to a closed state
for disconnecting the variable-volume chamber and the pressurizing
chamber from each other in the event of an abnormality of the
assisting device.
The volume of the variable-volume chamber can be controlled by the
fluid amount control device. However, if the volume of the
variable-volume chamber is reduced while the variable-volume
chamber is disconnected from the pressurizing chamber, the fluid
pressure in the variable-volume chamber is increased, Accordingly,
the fluid pressure in the braking fluid chamber in the brake
cylinder can be made higher than that in the pressurizing chamber,
by disconnecting the variable-volume chamber from the pressurizing
chamber, even in the event of abnormality of the assisting device,
that is, even if the fluid pressure in the pressurizing chamber
cannot be sufficiently increased with the assisting drive force
being zero or extremely small.
The assisting device may include the assisting cylinder,
high-pressure source, reservoir, solenoid-operated pressure control
valve device and control valve control device, as described above
with respect to the above mode (3), and the master cylinder
characteristic control device may include a master cylinder
characteristic control cylinder, and another set of a high-pressure
source, a reservoir, a solenoid-operated pressure control valve
device and a control valve control device, so that the volume of
the variable-volume chamber is controlled by the fluid pressure in
the master, cylinder characteristic control cylinder. In this case,
a single high-pressure source may be commonly used by the assisting
device and the master cylinder characteristic control device. If
the pressure of the pressurized fluid of the single high-pressure
source is lowered below a predetermined lower limit due to an
abnormality of the high-pressure source, it is difficult to
sufficiently pressurize the fluid in the variable-volume chamber
even when the emergency master cylinder disconnecting device is
switched to the closed state. In this case, it is difficult to
increase the fluid pressure in the brake cylinder to be higher than
that in the master cylinder pressure. However, the pressurized
fluid can be supplied from the high-pressure source to the
variable-volume chamber of the master cylinder characteristic
control device, while the high-pressure source is normal, even in
the event of an abnormality of the solenoid-operated pressure
control valve device or control valve control device of the
assisting device, a fluid leakage from the fluid passages connected
to the high-pressure source, reservoir, solenoid-operated pressure
control valve device and assisting pressure chamber, or a failure
of the assisting cylinder. Since the variable-volume chamber can be
pressurized by the high-pressure source, the fluid pressure in the
brake cylinder can be made higher than that in the pressurizing
chamber of the master cylinder.
Where two high-pressure sources are provided for the assisting
device and the master cylinder characteristic control device,
respectively, or where the assisting device includes the
high-pressure source described above while the master cylinder
characteristic control device includes an electric actuator such as
an electric motor to control the volume of the variable-volume
chamber, it is also effective to switch the emergency master
cylinder disconnecting device to the closed state in the event of
an abnormality of the high-pressure source of the assisting
device.
It is also effective to switch the emergency master cylinder
disconnecting device to the closed state in the event of an
abnormality of the assisting device, where the assisting device is
adapted to control the assisting drive force by controlling an
electric motor, as described above with respect to the above mode
(4) while the master cylinder characteristic control device
includes, the high-pressure source, or where the assisting device
and the master cylinder characteristic control device have
respective electric actuators.
(30) A hydraulically operated braking system comprising: (a) a
brake operating member operable by an operator; (b) a master
cylinder including a cylinder housing and a pressurizing piston
operatively connected to the brake operating member and cooperating
with the cylinder housing to define a pressurizing chamber, the
pressurizing piston being moved by the brake operating member to
pressurize a fluid in the pressurizing chamber; (c) a brake
cylinder actuated by the pressurized fluid received from the master
cylinder; and (d) a master cylinder characteristic control device
for controlling an amount of the fluid in the pressurizing chamber
of the master cylinder, to thereby control a relationship between a
position of the pressurizing piston relative to the cylinder
housing and the fluid pressure in the pressurizing chamber, for
controlling a fluid pressurizing characteristic of the master
cylinder.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and optional objects, features, advantages and technical
and industrial significance of this invent-ion will be further
clarified by reading the following detailed description of
presently preferred embodiments or forms of the invention, by
reference to the accompanying drawings, in which:
FIG. 1 is a circuit diagram showing a hydraulically operated
braking system constructed according to a first embodiment of this
invention;
FIG. 2 is a view schematically showing a brake pedal connected to a
master cylinder and an assisting cylinder 78 of an assisting device
in the braking system of FIG. 1;
FIG. 3 is a graph indicating a relationship between brake pedal
operating force and master cylinder fluid pressure controlled in
the braking system of FIG. 1;
FIG. 4 is a graph indicating a relationship between the brake pedal
operating stroke and the master cylinder fluid pressure controlled
in the braking system of FIG. 1;
FIG. 5 is a graph indicating a relationship between the master
cylinder fluid pressure and the power of brake pedal operating
force when the fluid pressures in assisting pressure chamber of the
assisting cylinder and volume control chamber of stroke adjusting
cylinder are controlled in the braking system of FIG. 1;
FIG. 6 is a graph indicating a relationship between the master
cylinder fluid pressure and the brake pedal operating stiffness
when the fluid pressures in the assisting pressure chamber and the
volume control chamber are controlled in the braking system of FIG.
1;
FIG. 7 is a graph indicating an operating characteristic of a
pressure switch provided in the assisting device of the braking
system of FIG. 1;
FIG. 8 is a flow chart illustrating a motor control routine
executed according to a program stored in a ROM of a pressure
control device of the braking system of FIG. 1, for controlling a
pump drive electric motor used in the assisting device;
FIG. 9 is a graph indicating a relationship between the vehicle
running speed and the master cylinder pressure controlled in the
braking system of FIG. 1;
FIG. 10 is a graph indicating a relationship between the brake
pedal operating speed and the master cylinder pressure controlled
in the braking system of FIG. 1;
FIG. 11 is an cross sectional view of a pressure increase control
valve included in the assisting device;
FIG. 12 is a circuit diagram, showing a hydraulically operated
braking system according to a second embodiment of this
invention;
FIG. 13 is a cross sectional view of a stroke adjusting cylinder
used in a hydraulically operated braking system according to a
third embodiment of this invention;
FIG. 14 is a circuit diagram showing a hydraulically operated
braking system according to a fourth embodiment of the
invention;
FIG. 15 is a circuit diagram showing a hydraulically operated
braking system according to a fifth embodiment of the
invention;
FIG. 16 is a view schematically showing a brake pedal connected to
a master cylinder in the braking system of FIG. 15;
FIG. 17 is a circuit diagram showing a hydraulically operated
braking system according to a sixth embodiment of the present
invention;
FIG. 18 is a circuit diagram showing a part of a hydraulically
operated braking system according to a seventh embodiment of this
invention;
FIG. 19 is a graph indicating a relationship between the master
cylinder pressure and the brake pedal operating stroke in the
braking system of FIG. 18;
FIG. 20 is a view showing a part of a hydraulically operated
braking system according to an eighth embodiment of the
invention;
FIG. 21 is a circuit diagram showing a hydraulically operated
braking system according to a ninth embodiment of the
invention;
FIG. 22 is a circuit diagram showing a part of a hydraulically
operated braking system according to a tenth embodiment of the
invention;
FIG. 23 is a view showing an example of control in various
conditions in the braking system of FIG. 22;
FIG. 24 is a circuit diagram showing a part of a hydraulically
operated braking system according to an eleventh embodiment of this
invention;
FIG. 25 is a view schematically showing a regulator provided in the
braking system of FIG. 24; and
FIG. 26 is a view showing an example of control in various
conditions in the braking system of FIG. 24.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring first to FIG. 1, the hydraulically braking system shown
therein includes a normal braking system and a servo system which
are independent of each other. The normal braking system includes a
brake operating member in the form of a brake pedal 10, a master
cylinder 12, and wheel brake cylinders 22, 24, 26, 28 for
respective wheels 14, 16, 18, 20 of an automotive vehicle. The
master cylinder 12 is a tandem type cylinder having two
pressurizing chambers 30 and 32. A pressure of a working fluid in
these pressurizing chambers 30, 32 are pressurized by an advancing
movement of a pressurizing piston 34 operatively connected to the
brake pedal 10. The fluid pressure in the pressurizing chambers 30,
32 changes with an operating force acting on the brake pedal 10.
Two fluid passages 36, 38 are connected to the respective
pressurizing chambers 30, 32.
Each of the fluid passages 36, 38 is split into two branches. The
wheel brake cylinders 22, 24 are connected to the ends of the two
branches of the fluid passage 36, while the wheel brake cylinders
26, 28 are connected to the ends of the two branches of the fluid
passage 38. In each of the four branches of the fluid passages 36,
38, there is provided a pressure holding valve in the form of a
solenoid-operated shut-off valve 44. The branches of each fluid
passage 36, 38 are connected to a reservoir 46 through fluid
passages 47. Four pressure reducing valves in the form of
solenoid-operated shut-off valves 48 are provided in the fluid
passages 47 so that the fluid can be discharged from the wheel
brake cylinders 22, 24, 26, 28 into the reservoirs 451 through the
respective pressure reducing shut-off valves 48 and the respective
fluid passages 47. A check valve 49 is provided in a by-pass
passage which by-passes each of the four pressure holding shut-off
valves 4. Each of the check valves 49 allows a fluid flow in a
direction from the corresponding wheel brake cylinder 22, 24, 26,
28 towards the master cylinder 12, and inhibits a fluid flow in the
opposite direction. The check valves 49 permit the fluid to be
rapidly returned from the wheel brake cylinders 22, 4, 26, 28 to
the master cylinder 12 when the brake pedal 10 is released from an
operated position towards the non-operated position while the
pressure holding shut-off valves 44 are placed in their closed
positions.
The two reservoirs 46 are connected to the respective fluid
passages 36, 38 also through respective fluid passages 50. In each
of the fluid passages-50, there are provided two check valves 52,
54, a pump 56 and a damper 58. The pump 56 is disposed between the
two check valves 52, 54. The damper 58 functions to reduce pressure
pulsation of the fluid delivered from the pump 56. The two pumps 56
are driven by a common electric motor 60.
In a portion of the fluid passage 36 between the pressurizing
chamber 30 and the point of branching, there is provided a
solenoid-operated shut-off valve 62. A stroke adjusting cylinder 64
is connected to a portion of the fluid passage 36 between the
shut-off valve 62 and the point of branching. These shut-off valve
62 and the stroke adjusting cylinder 64 will be described.
The servo system of the present hydraulically operated braking
system includes a pump 70, an accumulator 72, a pressure increase
control valve 74, a pressure reduction control valve 76, a master
reservoir 76 and an assisting cylinder 78. The pressure increase
control valve 74 and the pressure reduction control valve 75 are
controlled by a pressure control device 80 which is constituted
principally by a computer. It will be understood from the following
description that the above-indicated elements 70-78 and a portion
of the pressure control device 80 assigned to control the control
valves 74, 75 cooperate to constitute an assisting device 81 for
boosting a drive force to be applied to the pressurizing piston 32
of the master cylinder 10. It will also be understood that the
pressure increase control valve 74 and the pressure reduction
control valve 75 constitute a major portion of a solenoid-operated
pressure control valve device 82 of the assisting device 81.
The working fluid in the master reservoir 76 is pumped up and
pressurized by the pump 70, and the pressurized fluid is stored in
the accumulator 72. A pressure switch 83 is provided to monitor
whether the pressure or the fluid stored in the accumulator 72
falls within a predetermined range. An electric motor 84 for
driving the pump 70 is controlled according to an output signal of
the pressure switch 83, so that the fluid pressure in the
accumulator 72 is held substantially within the predetermined
range.
The pressure switch 83 has a plurality of switching portions, so
that the pressure switch 83 is turned when the fluid pressure in
the accumulator 72 (hereinafter referred to as "accumulator
pressure") is lowered below a predetermined lower limit, and is
turned ON when the accumulator pressure rises above a predetermined
upper limit, as indicated in the graph of FIG. 7. The electric
motor 84 is turned ON when the pressure switch 83 is turned OFF,
and is turned OFF when the pressure switch 83 is turned ON. The
electric motor 84 is kept operated while the pressure switch 83 is
OFF, so that the accumulator pressure is held within the
predetermined range defined by the lower and upper limits indicated
above.
The electric motor 84 is controlled by the pressure control device
80 depending upon the output signal of the pressure switch 83,
according to a motor control routine illustrated in the flow chart
of FIG. 8. This routine is initiated with step S1 to determine
whether the electric motor 84 is in operation. If a negative
decision (NO) is obtained in step S1, the control flow goes to step
S2 to determine whether the pressure switch 83 is in the ON state.
If the pressure switch 83 is in the ON state, the electric motor 84
is held OFF. If the pressure switch 83 is in the OFF state, that
is, if the accumulator pressure is lower than the lower limit, a
negative decision (NO) is obtained in step S2, and the control flow
goes to step S3 to turn ON the electric motor 84. If an affirmative
decision (YES) is obtained in step S1, that is, if the electric
motor 84 is in operation or in the ON state, step 14 is implemented
to determine whether the pressure switch 83 is in the ON state. If
the pressure switch 83 is in the ON state, that is, if the
accumulator pressure is higher than the upper limit, a negative
decision (NO) is obtained in step S3, and the control flow goes to
step S4 to turn OFF the electric motor 60. If an affirmative
decision (YES) is obtained in step S4, the electric motor 84 is
held in operation.
A pressure relief valve 86 is provided between the delivery side of
the pump 70 and the reservoir 76, to prevent an excessive rise of
the delivery pressure of the pump 70.
The assisting cylinder 78 includes a cylinder housing 90, an
assisting piston 92 fluid-tightly and slidably received in the
cylinder housing 90, and a return spring 94 disposed between the
assisting piston 92 and one of the opposite axial ends of the
cylinder housing 9G.
As shown in FIG. 2, the assisting piston 92 is operatively
connected to the brake pedal 10 by a piston rod 95. The brake pedal
10 has a fulcrum 96 at an intermediate portion thereof between the
lower end having a pedal pad 97 and the upper end. The piston rod
95 engages a portion of the brake pedal 10 between the fulcrum 96
and the upper end, more precisely, near the upper end. On the other
hand, the pressurizing piston 34 of the master cylinder 12 is
operatively connected by a piston rod 98 to a portion of the brake
pedal 10 between the brake pad 97 and the fulcrum 96. As indicated
in FIG. 2, the points of engagement of the rods 95, 98 with the
brake pedal 10 and the fulcrum 96 line on a straight line, and the
fulcrum 96 is spaced from the center of the pedal pad 97 by a
distance L.sub.F in the direction parallel to the straight line.
The point of engagement of the rod 98 with the brake pedal 10 is
spaced from the fulcrum 96 by a distance L.sub.M, while the point
of engagement of the rod 95 with the brake pedal 10 is spaced from
the fulcrum 96 by a distance L.sub.S.
The rods 98 of the pressurizing piston 34 of the master cylinder 12
and the rod 95 of the assisting piston 92 of the assisting cylinder
78 engage the brake pedal 10 such that the rods 98, 95 are
pivotable relative to the brake pedal 10 and such that the engaging
ends of the rods 98, 95 are movable relative to the brake pedal 10
in the longitudinal direction of the brake pedal 10 (in the
direction parallel to the straight line indicated above). To this
end, the brake pedal 10 has elongate two elongate holes 99a while
the rods 98, 95 have respective pins 99b engaging the respective
elongate holes 99a. In this arrangement, the engaging ends of the
rods 98, 95 are movable in the longitudinal direction of the brake
pedal 10 while maintaining the predetermined constant distances
L.sub.M, L.sub.S between the fulcrum 96 and the engaging ends.
Referring back to FIG. 1, the cylinder housing 90 and the assisting
piston 92 cooperate to define a spring chamber and an assisting
pressure chamber 100 on the opposite sides of the assisting piston
92. The return spring 94 is disposed in the spring chamber, which
communicates with the atmosphere. The assisting pressure chamber
100 communicates with the accumulator 72 and the master reservoir
76 through the pressure increase control valve 74 and the pressure
reduction control valve 75, respectively. The fluid pressure in the
assisting pressure chamber 100 is controllable by controlling the
control valves 74, 75.
The pressure increase and pressure reduction control valves 74 and
75 are identical in construction with each other. The following
description of the pressure increase control valve 74 substantially
applies to the pressure reduction control valve 75. The pressure
reduction control valve 74 is disposed between the delivery side of
the pump 70 and the assisting pressure chamber 100. As shown in
FIG. 11, the pressure reduction control valve 74 includes an
electromagnetic force generating device 103, and a spring 104. The
electromagnetic force generating device 103 includes a seating
valve 10L and a coil 102. The seating valve 10L has a valve member
105 and a valve seat 106. A force corresponding to a fluid pressure
difference across the seating valve 101 acts on the valve member
105 in a direction that causes the valve member 105 to be spaced
away from the valve seat 106. On the other hand, a biasing force of
the spring 104 acts on the valve member 105 in a direction that
causes the valve member 105 to be seated on the valve seat 106.
Further, an electromagnetic force generated by application of an
electric current to the coil 102 of the electromagnetic force
generating device 103 acts on the valve member 105 so as to move it
away from the valve seat 106. The electromagnetic force can be
controlled by controlling the electric current applied to the coil
102.
As described above, the force based on the fluid pressure
difference, the electromagnetic force and the biasing force of the
spring 104 act on the valve member 105 of the pressure increase
control valve 74. The force based on, the fluid pressure difference
and the electromagnetic force act on the valve member 105 in the
same direction, so as to move the valve member 104 away from the
valve seat 106, while the biasing force of the spring 104 acts on
the valve member 105 in the opposite direction, so as to move the
valve member 105 to be seated on the valve seat 100. Therefore, the
pressure increase control valve 74 is held open with the valve
member 105 spaced apart from the valve seat 106 while a sum of the
force based on the fluid pressure difference and the
electromagnetic Force is larger than the biasing force of the
spring 104. With the control valve 74 placed in the open state, the
pressurized fluid delivered from the pump 70 is permitted to flow
into the assisting pressure chamber 100 of the assisting cylinder
78, causing a rise of the fluid pressure in the chamber 100. The
fluid pressure difference required to hold the press-Lire increase
control valve 74 in the open state decreases with an increase in
the electromagnetic force, that is, with an increase in the
electric current applied to the coil 102.
Similarly, the pressure reduction control valve 75 is held open
while the above-indicated sum is larger than the biasing force of
the return spring 104. In this open state of the pressure reduction
control valve 75, the pressurized fluid is permitted to be
discharged from the assisting pressure chamber 100 into the master
reservoir 76, causing a reduction of the fluid pressure in the
chamber 100. The fluid pressure in the chamber 100 is reduced as
the amount of electric current applied to the coil 102 of the
control valve 75 is increased.
The return spring 94 of the assisting cylinder 78 is provided in
order to return the assisting piston 92 to its original position
when the brake pedal 10 is released. Since the piston 92 is
operatively connected to the brake pedal 10 through the rod 95, the
brake pedal 10 is also returned to its original or non-operated
position (fully released position) when the piston 92 is returned
by the return spring 94. In this respect, the return spring 94 also
functions to return the brake pedal 10. The non-operated position
or the brake pedal 10 is defined by a suitable stop member.
In the present first embodiment of this invention, a normally-open
solenoid-operated shut-off valve 108 is provided between the
assisting pressure chamber 100 and the pressurizing chamber 32 of
the master cylinder 12. While the assisting device 81 is normally
operable, this shut-off valve 108 is closed with an electric
current applied to its solenoid coil when the brake pedal 10 is
operated. While the assisting device 81 has any abnormality or
defect, the shut-off valve 108 is held closed even if the brake
pedal 10 is operated. The abnormality indicated above, which will
be referred to as "first kind of abnormality", is an electrical
defect associated with the assisting device 81, such as a failure
of normal operation of the electric motor 84, and a failure of
application of an electric current to the solenoid coil of the
pressure increase control valve 74 or pressure reduction control
valve 75. In the event of occurrence of the first kind of
abnormality, no electric current is applied to the solenoid coil of
the shut-off valve 108, whereby the shut-off valve 108 is opened.
Even in the presence of the first kind of abnormality, the
assisting cylinder 78 is normally operable.
A portion of the assisting de-vice 81 assigned to control the fluid
pressure in the assisting pressure chamber 100 functions as an
assisting drive force control device 109 for controlling an
assisting drive force which is generated by the assisting cylinder
78 and which is applied to the pressurizing piston 34. The first
kind of abnormality indicated above may be considered to be an
abnormality of the assisting drive force control device 109. This
assisting drive force control device 109 may be considered to be an
electrically operated hydraulic pressure source. The first kind of
abnormality may be determined to be present if the fluid pressure
in the assisting pressure chamber 100 is lowered below a
predetermined threshold, or if the pressure switch 83 is held in
the OFF state for more than a predetermined time. In this case, the
first kind of abnormality includes a failure to control the fluid
pressure in the assisting pressure chamber 100 due to a leakage of
the fluid from the hydraulic system.
The stroke adjusting cylinder 64 indicated above includes a
cylinder housing 110, and a volume-changing piston or stroke
adjusting piston 114 slidably received in the cylinder housing 110.
The cylinder housing 110 and the stroke adjusting piston 114
cooperate to define two fluid chambers 116, 118. The fluid chamber
116 is connected to the fluid passage 36 indicated above, while the
other fluid chamber 118 is connected to the accumulator 72 through
a pressure increase control valve 122 and to the master reservoir
76 through a pressure reduction control valve 124. The fluid
chamber 116 will be referred to as "a variable volume chamber",
while the fluid chaffer 118 will be referred to as "a volume
control chamber". A return spring 126 is disposed in the
variable-volume chamber 116, so that the stroke adjusting piston
114 is biased by the return spring 126 in a left direction as seen
in FIG. 1, namely, in the direction that causes an increase in the
volume or the variable-volume chamber 116.
The pressure increase control valve 122 and the pressure reduction
control valve 124 are identical in construction with the pressure
increase and pressure reduction control valves 74, 75 described
above. By controlling the electric currents applied to the solenoid
coils of these control valves 122, 124, the fluid pressure in the
volume control chamber 118 of the stroke adjusting cylinder 64 can
be controlled.
When the stroke adjusting piston 114 is moved in the right
direction as seen in FIG. 1 with an increase in the fluid pressure
in the volume control chamber 118, the volume of the
variable-volume chamber 116 decreases, causing an increase in the
fluid pressure in the variable-volume chamber 116, so that the
pressurized fluid is fed from the variable-volume chamber 116 into
the pressurizing chamber 30 of the master cylinder 12. When the
stroke adjusting piston 114 is moved in the left direction with a
decrease in the fluid pressure in the volume control chamber 118,
the fluid in the pressurizing chamber 30 is discharged into the
variable-volume chamber 116. Thus, the operating stroke of the
brake pedal 10 can be adjusted by controlling the fluid pressure in
the volume control chamber 118, that is, by controlling the fluid
pressure in the volume control chamber 118 to control the volume of
the fluid in the variable-volume chamber 114 when the brake pedal
10 is depressed, that is, when the pressurizing piston 34 is moved
in left direction. Described more specifically, the operating
stroke of the brake pedal 10 is reduced with an increase in the
amount of the fluid which is supplied from the variable-volume
chamber 116 to the pressurizing chamber 30 so as to move the
pressurizing piston 34 in the right direction. Conversely, the
operating stroke of the brake pedal 10 is increased with a decrease
in the amount of the fluid supplied to the pressurizing chamber
30.
As described above, the operating stroke of the brake pedal 10 can
be controlled by controlling the fluid pressure in the volume
control chamber 118 to control the volume of the variable-volume
chamber 116 and the amount of the fluid in the pressurizing chamber
30 when the brake pedal 10 is depressed. The original or neutral
position of the stroke adjusting piston 114 when the brake pedal 10
is in the non-operated or fully released position is determined by
equilibrium between a force acting on the piston 114 based on the
fluid pressure in the volume control chamber 118 and a biasing
force of the return spring 126 acting on the piston 114. It will be
understood that a stroke adjusting device 128 is constituted by the
stroke adjusting cylinder 64, pressure increase control valve 122,
pressure reduction control valve 124, accumulator 72, pump 70,
electric motor 84, master reservoir 76, and a portion of the
pressure control device 80 assigned to control the control valves
122, 124.
The solenoid-operated shut-off valve 62 is a normally-open valve,
which is held open while the assisting device 81 is normally
operable. When the assisting device 81 has any abnormality, that
is, when either of the pressure increase and pressure reduction
control valves 74, 7, has any abnormality, with the accumulator 72
and pump 70 being normally operable, the shut-off valve 62 is
closed. In this event, the fluid pressure in the assisting pressure
chamber 100 of the assisting cylinder 78 cannot be controlled, but
the fluid pressure in the volume control chamber 118 of the stroke
adjusting cylinder 64 can be controlled by controlling the pressure
increase and pressure reduction control valves 122, 124, since the
accumulator 72 and the pump 70 are normally operable. Accordingly,
the fluid pressure in the variable-volume chamber 116 can be made
higher than that in the pressurizing chamber 30, by reducing the
volume of the variable-volume chamber 116 by increasing the fluid
pressure in the volume control chamber 118. The abnormality of the
control valves 74, 75 indicated above will be referred to as "a
second kind of abnormality" of the assisting device 81. This second
kind of abnormality can be detected by detecting that the fluid
pressure in the assisting pressure chamber 100 is lower than a
predetermined lower limit even while the pressure switch 83 is in
the ON state. Even in the presence of the second kind of
abnormality, the fluid pressures in the wheel brake cylinders 22,
24 can be made higher than the fluid pressure in the pressurizing
chamber 30, owing to the fluid pressure in the variable-volume
chamber 116 of the stroke adjusting cylinder 64 which is
disconnected from the pressurizing chamber 30 by the shut-off valve
62. In this case, the fluid pressure in the pressurizing chamber 32
is applied to the wheel brake cylinders 26, 28.
The pressure control device 80 is principally constituted by a
computer incorporating a central processing unit (CPU) 130, a
random-access memory (RAM) 131, a read-only memory (ROM) 132, an
input portion 133 and an output portion 134. The input portion 133
receives output signals of wheel speed sensors 140, 142 144, 146
for the wheels 13, 16, 18, 20, a force sensor 148 for detecting the
operating force F acting on the brake pedal 10, a stroke sensor 150
for detecting the operating stroke S of the brake pedal 10, an
assisting pressure sensor 152 for detecting the fluid pressure in
the assisting pressure chamber 100, a stroke control pressure
sensor 154 for detecting the fluid pressure in the
variable-volatile chamber 118, and a master cylinder pressure
sensor 156 for detecting the fluid pressure in the pressurizing
chamber 30 of the master cylinder 12.
To the output portion 134, there are connected driver circuits for
energizing the solenoid coils of the pressure increase control
valves 74, 122, pressure reduction control valves 75, 124, pressure
holding and increasing shut-off valves 44, 48, and shut-off valves
62, 108, and driver circuits for energizing the electric motors 60,
84. The ROM 132 stores various control programs including the
control program for the motor control routine illustrated in the
flow chart of FIG. 8 described above, an anti-lock braking pressure
control program, an assisting drive force control program for
controlling the fluid pressure in the assisting pressure chamber
100, a stroke adjusting control program for controlling the fluid
pressure in the volume control chamber 118, and control programs
for controlling the various solenoid-operated shut-off valves such
as the shut-off valves 62, 74, 75, 108, 122, 124. The ROM 132
further stores control data maps represented by the graphs of FIGS.
3 and 4, which are used to control the fluid pressure in the
assisting pressure chamber 100.
The force sensor 148 detects the operating or depression force F
acting on the pedal pad 97 of the brake pedal 10. For instance, the
force sensor 148 uses an elastic meter attached to the pedal pad
97, so that the operating force F is detected based on an amount of
elastic deformation or strain of the elastic member.
The master cylinder pressure sensor 156, which detects the fluid
pressure in the pressurizing chamber 30, is disposed in a portion
of the fluid passage 36 which is located downstream of the
solenoid-operated shut-off valve 62. Accordingly, the pressure
sensor 156 cannot detect the fluid pressure in the chamber 30 when
the shut-off valve 62 is in the closed position. In this case, se
output of the pressure sensor 156 represents the fluid pressure
applied to the wheel brake cylinders 22, 24.
An assisting pressure force acting on the assisting piston 92 can
be obtained on the basis of the fluid pressure in the assisting
pressure chamber 100. Based on this assisting pressure force, an
assisting drive force is applied to the pressurizing piston 34
through the brake pedal 10. On the other hand, the operating stroke
S of the brake pedal 10 is determined by the amounts of the fluid
in the pressurizing chamber 30 and the variable-volume chamber 116,
which amounts are determined by the fluid pressure in the volume
control chamber 118 and the pressurizing chamber 30. In the present
embodiment, the fluid pressure in the assisting pressure chamber
100 is controlled so that the master cylinder pressure P.sub.M
(fluid pressure in the pressurizing chamber 30) changes with the
operating force F of the brake pedal 10, according to a
predetermined relationship between the master cylinder pressure
P.sub.M and the operating force F, as indicated in the graph of
FIG. 3. On the other hand, the fluid pressure in the volume control
chamber 118 is controlled so that the master cylinder pressure
P.sub.M changes with the operating stroke S of the brake pedal 10,
according to a predetermined relationship between the master
cylinder pressure P.sub.M and the operating stroke S, as indicated
in the graph of FIG. 4.
On the basis of the rotating speeds of the wheels 14-20 detected by
the wheel speed sensors 140-146, an estimated running speed of the
automotive vehicle is obtained, and the slipping speed or locking
condition of each wheel is estimated on the basis of the detected
wheel speeds and the estimated vehicle running speed. The pressure
holding and pressure reducing shut-off valves 44, 48 are controlled
to control the fluid pressures in the wheel brake cylinders 22-28
in an anti-lock fashion, according to the estimated locking
conditions of the wheels 14-20.
An operation of the present hydraulically operated braking system
constructed as described above will be described.
When the brake pedal 10 is depressed with the operating force F,
the solenoid-operated shut-off valve 108 is brought to the closed
position while the solenoid-operated shut-off valve 62 is held in
its open position, where the assisting device 81 is normal. The
fluid pressure in the assisting pressure chamber-100 is controlled
to control the assisting drive force. The fluid pressure in the
volume control chamber 118 has been controlled to adjust the
operating stroke S of the brake pedal.
Initially, an operation to control the fluid pressure P.sub.S in
the assisting pressure chamber 100 to control the fluid pressure
P.sub.M in the pressurizing chambers 20, 32 will be described, by
reference to FIG. 2. In the present embodiment, the pressurizing
piston 34 has a pressure-receiving surface area S.sub.M which is
larger than a pressure-receiving surface area S.sub.S of the
assisting piston 92. That is, S.sub.M >S.sub.S. Further, a
product of the pressure-receiving surface area S.sub.M and the
distance L.sub.M (between the fulcrum 96 and the engaging end of
the rod 98) is larger than a product of the pressure-receiving
surface area S.sub.S and the distance L.sub.S (between the fulcrum
96 and the engaging end of the rod 95). That is,
S.sub.M.times.L.sub.M >S.sub.S.times.L.sub.S.
An equilibrium of moments about the fulcrum 96 of the brake pedal
10 is expressed by the following equation:
The force F.sub.S is the assisting pressure force acting on the
assisting piston 92 based on the fluid pressure P.sub.S in the
assisting pressure chamber 100. This assisting pressure force
F.sub.S is applied to the brake pedal 10. The force F.sub.M, which
is a wheel braking force, is a reaction force which is applied to
the brake pedal 10 based on the fluid pressure P.sub.M in the
pressurizing chambers 30, 32. The wheel braking force F.sub.M is
expressed by the following equation:
The first term (F.sub.S.times.L.sub.S /L.sub.M) in the right member
of the above equation is a force to be applied to the pressurizing
piston 34 based on the assisting pressure force F.sub.S. The force
(F.sub.S.times.L.sub.S /L.sub.M) is referred to as the assisting
drive force. On the other hand, the second term (F.times.L.sub.F
/L.sub.M) is a force to be applied to the pressurizing piston 34
based on the operating force F. The force (F.times.L.sub.F
/L.sub.M) is referred to as the primary drive force. Thus, the
assisting drive force in addition to the primary drive force is
applied to the pressurizing piston 34, so that the wheel braking
force is made larger in the present braking system than that in a
braking system not provided with the assisting device 81.
On the other hand, the assisting pressure force F.sub.S is
expressed by the following equation:
The wheel braking force F.sub.M is also expressed by the following
equation:
Therefore, the fluid pressure P.sub.M in the master cylinder 12 is
expressed by the following equation:
The master cylinder pressure P.sub.M is determined on the basis of
the detected operating force F and according to the P.sub.M -F
relationship of FIG. 3 represented by a data map stored in the ROM
132. Accordingly, a target value of the fluid pressure P.sub.S in
the assisting pressure chamber 100 is determined according to the
above equation, and the pressure increase control valve 74 and the
pressure reduction control valve 75 are controlled so as to
establish the determined target value of the fluid pressure
P.sub.S.
When the fluid pressure P.sub.S in the assisting pressure chamber
100 is .alpha. times the fluid pressure P.sub.M of the master
cylinder 12, that is, when P.sub.S =.alpha..times.P.sub.M, the
master cylinder pressure P.sub.M and the wheel braking force
F.sub.M are expressed by the following equations, respectively:
It will be understood that the boosting ratio of the master
cylinder pressure P.sub.M and the wheel braking force F.sub.M
increase with the value of .alpha..
As described above, the fluid pressure P.sub.S in the assisting
pressure chamber 100 is controlled so that the master cylinder
pressure P.sub.M increases with the operating force F, according to
the predetermined P.sub.M -F relationship as indicated in the graph
of FIG. 3. According to this relationship, the master cylinder
pressure P.sub.M increases with the operating force F according to
an equation P.sub.M =k.times.F.sup.1/2 or P.sub.M
=k.times.F.sup.2/3, while the operating force F is considerably
small. This arrangement permits a rapid rise of the master cylinder
pressure P.sub.M and a rapid increase of the wheel braking force.
While the operating force F is relatively large, the master
cylinder pressure P.sub.M increases as a quadratic function of the
operating force F, that is, according to an equation P.sub.M
=k.times.F.sup.2, so that the rate of increase of the master
cylinder pressure P.sub.M (wheel braking force) with the operating
force F is comparatively low while the operating force F is in a
medium range as indicated in FIG. 3, and so that the rate of
increase of the master cylinder pressure P.sub.M is comparatively
high while the operating force F is considerably large. In this
arrangement, the accuracy of control of the wheel braking force is
relatively high, but the braking sensitivity is relatively low,
while the operating force F is in the medium range. While the
operating force F is considerably large, the wheel braking force is
sufficiently large, and the braking sensitivity is relatively high,
that is, the amount of increase of the wheel braking force per a
given amount of increase of the operating force F is relatively
large.
The fluid pressure in the volume control chamber 118 of the stroke
adjusting cylinder 64 is controlled so that the master cylinder
pressure P.sub.M increases with the operating stroke S according to
the predetermined P.sub.M -S relationship as indicated in the graph
of FIG. 4. According to this relationship, the master cylinder
pressure P.sub.M increases as a quadratic function of the operating
stroke S, namely, according to an equation P.sub.M
=k'.times.S.sup.2. In this arrangement, the accuracy of control of
the wheel braking force is relatively high while the operating
stroke S is relatively small, and the rate of increase of the
master cylinder pressure P.sub.M with the operating stroke S is
relatively high while the operating stroke S is relatively large.
The operating stroke S is detected by the stroke sensor 150.
It is also noted that a relationship between the master cylinder
pressure P.sub.M and a power (efficiency) of the operating force F,
and a relationship between the master cylinder pressure P.sub.M and
an operating stiffness of the brake pedal 10 can be controlled by
controlling the relationship between the master cylinder pressure
P.sub.M and the operating force F and the relationship between the
master cylinder pressure P.sub.M and the operating stroke S.
The power of the operating force F is represented by dP.sub.M
/(F.times.dS+S.times.dF), while the operating stiffness is
represented by dF/dS. The power dP.sub.M /(F.times.dS+S.times.dF)
and the operating stiffness dF/dS will be considered in a case
where the fluid pressure P.sub.S in the assisting pressure chamber
100 is controlled so that the master cylinder pressure P.sub.M
increases with the operating force F according to the equation
P.sub.M =k.times.F.sup.2, while the fluid pressure in the volume
control chamber 118 is controlled so that the master cylinder
pressure P.sub.M increases with the operating stroke S according to
the equation P.sub.M =k'.times.S.sup.2. In these equations, k and
k' are constants, which may be changed as desired by controlling
the fluid pressure P.sub.S in the assisting pressure chamber 100.
In the present case, the power dP.sub.M /(F.times.dS+S.times.dF) is
held constant at .check mark.(kk'), as indicated in the graph of
FIG. 5, and the operating stiffness dF/dS is also held constant at
.check mark.(k'/k).
By controlling the fluid pressure P.sub.S in the assisting pressure
chamber 100 and the fluid pressure in the volume control chamber
118 of the stroke adjusting cylinder 64 according to the above
equations P.sub.M ==k.times.F.sup.2, and P.sub.M =k'.times.S.sup.2,
as described above, the power dP.sub.M (F.times.dS+S.times.dF) is
held constant, so that the rate of increase of the wheel braking
force with the braking effort of the vehicle operator applied to
the brake pedal 10 is held constant, making it possible to give the
vehicle operator a brake operating feel that the braking effect
increases linearly with an increase in the braking effort by the
vehicle operator. Further, the operating stiffness dF/dS is also
held constant, so that the vehicle operator operating the brake
pedal 10 has a highly consistent brake operating feel. In addition,
the constant values .check mark.(kk') and .check mark.(k'/k) of the
power and operating stiffness can be changed to change the braking
effect characteristic, by changing the constants k and k'.
Then, the power dP.sub.M /(F.times.dS+S.times.dF) and the operating
stiffness dF/dS will be considered in another case where the fluid
pressure P.sub.S in the assisting pressure chamber 100 and the
fluid pressure, in the volume control chamber 118 are controlled so
that the master cylinder pressure P.sub.M increases with the
operating force F and the operating stroke S according to the
equations P.sub.M =k.times.F.sup.2/3 and P.sub.M =k'.times.S.sup.2,
respectively. In this case, the power changes as a function of
P.sub.M.sup.-1, that is, the power is represented by {.check
mark.(k.sup.3 k')/2P.sub.M }, while the operating stiffness changes
as a function of P.sub.M, as indicated in FIG. 6, that is, the
operating stiffness is represented by {3.check
mark.(k'/k.sup.3).times.P.sub.M }.
By controlling the fluid pressure P.sub.S in the assisting pressure
chamber 100 and the fluid pressure P.sub.S in the volume control
chamber 118 of the stroke adjusting cylinder 64 according to the
above equations P.sub.M ==k.times.F.sup.2/3, and P.sub.M
=k'.times.S.sup.2, as described above, the power dP.sub.M
/(F.times.dS+S.times.dF) is larger when the wheel braking force is
relatively small than when it is relatively large, so that the rate
of increase of the wheel braking force with the braking effort of
the vehicle operator applied to the brake pedal 10 is higher when
the wheel braking force (braking effort) is relatively small than
when it is relatively large. Accordingly, the vehicle operator
feels a relatively high rate of increase of the braking effect with
an increase in the braking effort applied to the brake pedal 10.
Further, the operating stiffness of the brake pedal 10 increases
with the braking effort at a relatively high rate, so that the
vehicle operator feels a relatively high degree of operating
stiffness of the brake pedal 10.
The power and the operating stiffness will be further considered in
a case where the fluid pressure P.sub.S in the assisting pressure
chamber 100 and the fluid pressure in the volume control chamber
118 are controlled so that the master cylinder pressure P.sub.M
changes as ordinary quadratic functions of the operating force F
and the operating stroke S, that is, according to the following
equations, respectively
In the above equations, a, b, c and d are constants, which can be
changed as desired by controlling the fluid pressure P.sub.S in the
assisting pressure chamber 100.
In the present case, the operating stiffness dF/dS is represented
by .check mark.{k'(P.sub.M -d)/k(P.sub.M -b)}. Where the constant b
is equal to the constant d, the value .check mark.{k'(P.sub.M
-d)/k(P.sub.M -b)} is constant at .check mark.(k'/k). Further, the
power dP.sub.M /(F.times.dS+S.times.dF) is represented by 2.check
mark.(kk')XY/(X.sup.2 +Y.sup.2 +a.check mark.kX+c.check mark.k'Y),
where X represents .check mark.(P.sub.M -b) while Y represents
.check mark.(P.sub.M -d). Where the constants a and c are zero, and
X is equal to Y (b=d), the value 2.check mark.(kk')XY/(X.sup.2
+Y.sup.2 +a.check mark.kX+c.check mark.k'Y) is constant at .check
mark.(kk'). Where the constants a, c and d are zero, the power is
represented by 2.check mark.(kk).check mark.{P.sub.M (P.sub.M
-b)}/(2P.sub.M -b).
The power and the operating stiffness will be further considered in
a case where the fluid pressure P.sub.S in the assisting pressure
chamber 100 and the fluid pressure in the volume control chamber
118 are controlled so that the master cylinder pressure P.sub.M in
changes with the operating force F and the operating stroke S,
according to the following equations, respectively:
In the present case, the operating stiffness dF/dS is represented
by the following equation:
The above equation can be converted into the following equation
according to the Taylor's Theorem:
It will thus be understood that the operating stiffness dF/dS
changes linearly as a function of the master cylinder pressure
P.sub.M. This is true even where the constant d is equal to zero.
Accordingly, the operating stiffness increases with an increase in
the wheel braking force.
On the other hand, the power dP.sub.M /(F.times.dS+S.times.dF) is
represented by 2k.check mark.(kk')M/N.sup.3 +ak.sup.3/2 +3c.check
mark.k'MN+3M.sup.2 N), where M represents .check mark.(P.sub.M -d)
while N represents .check mark.P.sub.M. Where the constants c and d
are zero, the power is represented by 2k.check mark.(kk')/(4P.sub.M
+ak.sup.3/2 /.check mark.P.sub.M).
While the present embodiment is adapted to control the fluid
pressures in the assisting pressure chamber 100 and volume control
chamber 118 according to the specific P.sub.M -F and P.sub.M -S
relationships shown in FIGS. 3 and 4, it is to be understood that
the fluid pressures may be controlled according to other
relationships. For instance, a plurality of different P.sub.M -F
and/or P.sub.M -S relationships may be used corresponding to
respective ranges of the operating force F and/or operating stroke
S.
It is also possible to control the fluid pressures in the assisting
pressure chamber 100 and volume control chamber 118, depending upon
the vehicle running speed and the operating speed of the brake
pedal 10, rather than the operating force F and stroke S. For
instance, the fluid pressure P.sub.S in the assisting pressure
chamber 100 may be controlled so that the master cylinder pressure
P.sub.M (wheel braking force) increases with the vehicle running
speed V, such that the rate of increase of the pressure P.sub.M
increases with an increase of the vehicle running speed V, as
indicated by solid line in the graph of FIG. 9, in which broken
line indicates the P.sub.M -V relationship where the pressure
P.sub.S is controlled so that the master cylinder pressure P.sub.M
increases with the operating force F according to the P.sub.M -F
relationship as indicated in the graph of FIG. 3. In the case of
control indicated by the solid line of FIG. 9, the wheel braking
force is sufficiently increased when the vehicle is running at a
relatively high speed, making it possible to reduce the required
stopping distance of the vehicle upon brake application at the
relatively high speed. The vehicle running speed V used may be a
speed during or upon brake application to the vehicle.
Further, the fluid pressure P.sub.S may be controlled so that the
master cylinder pressure P.sub.M (wheel braking force) increases
with the operating speed dF/dt of the brake pedal 10, such that the
rate of increase of the pressure P.sub.M increases with an increase
in the operating speed dF/dt, as indicated by solid line in the
graph of FIG. 10. When the operating speed dF/dt of the brake pedal
10 is relatively high, it means an emergency brake application, or
a desire of the vehicle operator to abruptly stopping or
decelerating the running vehicle. The operating speed dF/dt may be
obtained on the basis of a rate of change of the level of the
output signal of the force sensor 148, or alternatively on the
basis of a rate of change of the master cylinder pressure P.sub.M
while the fluid pressure P.sub.S in the assisting pressure chamber
100 is held constant. Further, the operating speed dF/dt may be
obtained on the basis of a rate of change of the level, of the
output signal of the stroke sensor 150. In this case, however, it
is desirable to obtain the operating speed while the fluid pressure
in the volume control chamber 118 is held constant.
It is also possible to control the fluid pressures in the assisting
pressure chamber 100 and volume control chamber 118, depending upon
the friction coefficient .mu. of the road surface and/or the
viscosity of the working fluid. For instance, the fluid pressure
P.sub.S may be controlled so that the rate of increase of the wheel
braking force with the friction coefficient .mu. decreases with a
decrease in the friction coefficient .mu. or so that the assisting
drive force F.sub.S.times.L.sub.S /L.sub.M decreases with a
decrease in the friction coefficient .mu.. Further, the fluid
pressure P.sub.S may be controlled so that the assisting drive
force increases to increase the master cylinder pressure P.sub.M
with an increase in the viscosity of the working fluid, since the
force transmitting velocity decreases with the increase in the
fluid viscosity.
Further, both of the fluid pressures in the assisting pressure
chamber 100 and volume control chamber 118 need not be controlled.
Namely, only the fluid pressure in the assisting pressure chamber
110 or the volume control chamber 118 may be controlled.
It is noted that upon releasing of the brake pedal 10, the fluid in
the assisting pressure chamber 100 is returned to the master
reservoir 76 through the pressure reduction control valve 75. That
is, the control valve 75 is held in the open position for a
predetermined time after the brake pedal 10 has been released.
It is also noted that the assisting device 81 may be used as an
automatic braking device, which is automatically activated without
an operation of the brake pedal 10, when a predetermined condition
is satisfied. For instance, where the vehicle has a sensor for
detecting a distance between the vehicle front and any object such
as a person in front of the vehicle, the fluid pressure P.sub.S in
the assisting pressure chamber 100 is raised to apply a brake to
the vehicle to avoid a collision of the vehicle with the object,
when the detected distance has become smaller than a predetermined
threshold. In this case, the force based on the fluid pressure in
the assisting pressure chamber 100 is transmitted to the
pressurizing piston 34 through the brake pedal 10, so that the
piston 34 is automatically advanced to increase the master cylinder
pressure P.sub.M, namely, the pressure in the pressurizing chambers
30, 32, for applying an automatic brake to the vehicle, without the
vehicle operator depressing the brake pedal 10.
On the other hand, an anti-lock braking pressure control operation
is performed by controlling the solenoid-operated pressure holding
and reducing shut-off valves 44, 48, so as to regulate the fluid
pressure in each of the wheel brake cylinders 22, 24, 26, 28 so
that the amount of slip of each wheel 14, 16, 18, 20 is held within
an optimum value. The anti-lock braking pressure control operation
for each wheel is initiated when the amount of slip of the wheel on
the road surface during brake application to the vehicle has become
excessive with respect to the friction coefficient of the road
surface. During the anti-lock braking pressure control operation,
the fluid pressure P.sub.S in the assisting pressure chamber 100 is
controlled to be held at a predetermined level which is low enough
to reduce an influence of the pressure .sub.S on the anti-lock
braking pressure control operation.
There will next be described an operation of the present braking
system in the event of occurrence of an abnormality of the
assisting device 81. When the first kind of abnormality of the
assisting device 81 explained above occurs, the solenoid-operated
shut-off valve 108 is brought to its open state, and the
solenoid-coils of the pressure increase control valve 74 and
pressure reduction control valve 75 are de-energized, so that the
assisting pressure chamber 100 is disconnected from both of the
accumulator 72 and the master reservoir 76, and is communicated
with the pressurizing chamber 32. When the brake pedal 10 is
depressed in this condition, the pressurized fluid in the
pressurizing chamber 32 is supplied to the assisting pressure
chamber 100, and the assisting piston 92 is moved when the brake
pedal 10 is released, the pressurized fluid is returned from the
assisting pressure chamber 100 back to the pressurizing chamber 32,
and to the master reservoir 76. If the pressurizing chamber 32 were
not communicated with the assisting pressure chamber 100 through
the shutoff valve 108, the fluid flows into and from the assisting
pressure chamber 100 would be inhibited, preventing a movement of
the assisting piston 92, and therefore a movement of the brake
pedal 10. In the present embodiment wherein the shut-off valve 108
is opened in the event of occurrence of the first kind of
abnormality of the assisting device 81, the brake pedal 10 can be
depressed even in that event.
With the assisting pressure chamber 100 held in communication with
the pressurizing chamber 32 through the open solenoid-operated
shut-off valve 108, the fluid pressure in the assisting pressure
chamber 100 is made equal to the fluid pressure P.sub.M ' in the
pressurizing chamber 32. The fluid pressure P.sub.M ' is
represented by the following equation:
In the present embodiment wherein the inequality
L.sub.M.times.S.sub.M >L.sub.S.times.S.sub.S is satisfied, the
fluid pressure P.sub.M ' is prevented from being a negative
pressure. That is, although the communication between the
pressurizing chamber 32 and the assisting pressure chamber 100
causes the working fluid to flow from the pressurizing chamber 32
into the assisting pressure chamber 100, the fluid pressure in the
pressurizing chamber 32 will not fall below the atmospheric
pressure, so that the fluid will not be discharged from the wheel
brake cylinders 26, 28 into the pressurizing chamber 32.
Accordingly, the wheel brake cylinders 26, 28 can be actuated by
the fluid pressure pressurized in the pressurizing chamber 32.
The fluid pressure P.sub.M in the pressurizing chamber 32 when the
assisting drive force is zero is represented by the following
equation:
Therefore, the fluid pressure P.sub.M ' when the pressurizing
chamber 32 is in communication with the assisting pressure chamber
100 is represented by the following equation including the fluid
pressure P.sub.M :
Thus, a ratio of the fluid pressure P.sub.M ' when the shut-off
valve 108 is open to the fluid pressure P.sub.M when the assisting
drive force is zero is represented by the following equation:
Since the value (L.sub.S.times.S.sub.S /L.sub.M.times.S.sub.M) is
smaller than 1 as described above, it will be apparent that the
above-indicated ratio P.sub.M '/P.sub.M is larger than 1. Namely,
the wheel braking force when the pressurizing chamber 32 is in
communication with the assisting pressure chamber 100 in the event
of the first kind of abnormality of the assisting device 81 is
larger than when the assisting drive force is zero.
On the other hand, the solenoid-operated shut-off valve 62 is held
in the open state, and the fluid pressure in the volume control
chamber 118 is held constant. In the presence of an electrical
abnormality the solenoid coils of the pressure increase control
valve 122 and the pressure reduction control valve 124 are
de-energized, and the stroke adjusting cylinder 64 is not operable
to adjust the operating stroke S.
In the event of occurrence or the second kind of abnormality of the
assisting device 81 described above, the shut-off valve 62 is
brought to the closed state, and the solenoid coils of the pressure
increase and pressure reduction control valves 74, 75 are
de-energized, while the shut-off valve 108 is brought to the open
state as in the event of occurrence of the first kind of
abnormality. In this condition, the fluid pressure in the
variable-volume changer 116 can be made higher than that in the
pressurizing chamber 32, by controlling the fluid pressure in the
volume control chamber 118 while the variable-volume chamber 116 is
disconnected from the master cylinder 12 by the closed shut-off
valve 62. Thus, the fluid pressure in the wheel brake cylinders 22,
24 can be made higher than the fluid pressure in the master
cylinder 12. In this sense, the stroke adjusting cylinder 64 also
functions as a device for increasing the fluid pressure in the
wheel brake cylinders 22, 24 in the event of occurrence of the
second kind of abnormality.
The shut-off valve 62 may be brought to the closed state also when
the assisting cylinder 78 has an abnormality, such as a failure to
move the assisting piston 92. In this case, too, the pressurized
fluid can be supplied from the stroke adjusting cylinder 64 to the
wheel brake cylinders 22, 24. This abnormality can be detected if
the master cylinder pressure P.sub.M or the assisting pressure
drive force F.sub.S is lower or smaller than a predetermined
threshold while the operating force F is larger than a
predetermined value. In this case, it is desirable to hold the
shut-off valve 108 in the closed state, for preventing the fluid to
be discharged from the pressurizing chamber 32 into the assisting
pressure chamber 100.
As described above, the present hydraulically operated braking
system is constructed to electrically control the fluid pressure in
the assisting pressure chamber 100, permitting an electrical
control of the assisting drive force to be applied to the
pressurizing piston 34 of the master cylinder 12, so that the
master cylinder pressure P.sub.M can be controlled to a level in a
non-proportional relationship with the operating force F of the
brake pedal 10. That is, the relationship between the master
cylinder pressure and the brake pedal operating force can be
changed as desired. Further, the present braking system is equipped
with the stroke adjusting device 128 including the stroke adjusting
cylinder 64 having the volume control chamber 118 whose fluid
pressure can also be electrically controlled, so that the
relationship between the master cylinder pressure and the brake
pedal operating stroke can also be changed as desired. The stroke
adjusting device 128 can be utilized as a device for activating the
wheel brake cylinders 22, 24 with a relatively high fluid pressure,
in the event of abnormality of the assisting device 81. The
utilization of the stroke adjusting device 128 makes it possible to
apply a relatively high braking pressure to the wheel brake
cylinders 22, 24, without increasing the structural complexity of
the braking system. Even if the pressure increase and pressure
reduction control valves 74, 74 are not normally operable, the
electrical control to close the shut-off valve 62 permits the
stroke adjusting cylinder 64 to activate the wheel brake cylinders
22, 24 with the pressurized fluid supplied from the accumulator 72.
Further, the electrical control to open the shut-off valve 108 in
the event of an abnormality of the assisting drive force control
device 109 permits the brake pedal operating force to be
boosted.
It will be understood from the foregoing description of the present
first embodiment of the invention that a portion of the pressure
control device 80 assigned to control the pressure increase and
pressure reduction control valve 74, 75 constitutes a major portion
of a control valve control device for controlling the control
valves 74, 75, while the solenoid-operated shut-off valve 108 and a
portion of the pressure control device 80 assigned to open the
shut-off valve 108 constitute an emergency fluid communicating
device for effecting fluid communication between the pressurizing
chamber 32 and the assisting pressure chamber 100 in the event of
an abnormality of the assisting device 81. Since the fluid
pressurizing characteristic of the master cylinder 12 is controlled
by adjusting the operating stroke S by the stroke adjusting device
128, the stroke adjusting device 128 may be considered to be one
form of a master cylinder characteristic control device for
controlling the fluid pressurizing characteristic of the master
cylinder 12. Since the fluid pressurizing characteristic of the
master cylinder 12 can also be controlled by controlling the
assisting drive force produced by the assisting device 81, the
assisting device 81 including the assisting drive force control
device 108 may be considered to be another form of the master
cylinder characteristic control device. While both of the assisting
device 81 and the stroke adjusting device 128 may be considered to
be the master cylinder characteristic control device, each of these
two de-vices 81, 128 may be considered to be the master cylinder
characteristic control device, since either the device 81 or the
device 128 alone can change the fluid pressurizing characteristic
of the master cylinder 12. It will further be understood that the
stroke adjusting cylinder 64 having the volume control chamber 118
and a portion of the pressure control device 80 assigned to control
the fluid pressure in the chamber 118 constitute a master cylinder
fluid amount control device for controlling the amount of the fluid
in the master cylinder 12 to adjust the operating stroke S of the
brake pedal 10, and that the shut-off valve 62 and a portion of the
pressure control device 80 assigned to close the shut-off valve 62
constitute an emergency master cylinder disconnecting device for
disconnecting the variable-volume chamber 116 and the master
cylinder 12 from each other in the event of an abnormality of the
assisting drive force control device 109.
In the present embodiment, the fluid pressures in the assisting
pressure chamber 100 and volume control chamber 118 are controlled
so that the master cylinder pressure P.sub.M changes in the
predetermined relationships with the operating force F and stroke S
as indicated in the graph of FIGS. 3 and 4, respectively. However,
the fluid pressures may be controlled so that the deceleration
value of the vehicle during an operation of the brake pedal 10
coincides with a value corresponding to the operating force F and
stroke S. In this case, the braking system is provided with a
sensor for detecting the vehicle deceleration value. The vehicle
deceleration sensor may be a sensor for detecting the fluid
pressure in the wheel brake cylinders. That is, the fluid pressures
in the chambers 100, 118 may be controlled in a predetermined
relationship with the fluid pressure in the wheel brake
cylinders.
The present embodiment is further arranged such that the fluid
pressure in the volume control chamber 118 of the stroke adjusting
cylinder 64 while the brake pedal 10 is in the non-operated
position is held at a value necessary to hold the stroke adjusting
piston 114 at its original position. That is, the fluid pressure in
the chamber 118 when the brake pedal 10 is in the non-operated
position is determined such that a force acting on the piston 114
based on that fluid pressure is equal to the biasing force of the
return spring 126. However, a spring whose biasing force is equal
to that of the return spring 126 may be disposed on the volume
control chamber 118 to hold the piston 114 at its original position
when the brake pedal 10 is in the non-operated position. In this
case, the fluid pressure in the chamber 118 may be held at the
atmospheric pressure while the brake pedal 10 is in the
non-operated position. Described in detail, the fluid pressure in
the chamber 118 is lowered to the atmospheric pressure upon
releasing of the brake pedal 10, by holding the pressure reduction
control solenoid-operated shut-off valve 124 in its fully open
position with the maximum electric current applied to its solenoid
coil, for a predetermined time after the releasing or the brake
pedal 10, to return the fluid from the chamber 118 to the master
reservoir 76. This arrangement eliminates a need of controlling the
fluid pressure in the chamber 118 so as to hold the piston 114 at
its original position against the biasing force of the return
spring 126 after the brake pedal 10 is released.
Referring next to FIG. 12, a hydraulically operated braking system
according to a second embodiment of this invention will be
described. In this braking system, the variable-volume chamber 116
of the stroke adjusting cylinder 64 is connected to the
pressurizing chamber 30 of the master cylinder 12 through a fluid
passage 165, and to the fluid passage 36 through a fluid passage
166. A normally-open solenoid-operated shut-off valve 168 is
provided in the fluid passage 165. In the event of occurrence of
the second kind of abnormality of the assisting device 81, the
shut-off valve 168 is closed, to disconnect the variable-volume
chamber 116 from the pressurizing chamber 30, so that the fluid
pressure in the chamber 116 can be made higher than the fluid
pressure in the pressurizing chamber 30 upon depression of the
brake pedal 10, in order to activate the wheel brake cylinders 22,
24 with the pressurized fluid supplied thereto through the fluid
passages 166, 63.
In a third embodiment of the invention, a stroke adjusting cylinder
170 as shown in FIG. 3 is used in place of the stroke adjusting
cylinder 64. This stroke adjusting cylinder 170 includes a cylinder
housing 172, and a volume-changing piston or stroke adjusting
piston 117 slidably received in the cylinder housing 172. Described
more specifically, the cylinder housing 172 has a stepped bore
consisting of a small-diameter portion 175 and a large-diameter
portion 176 having a larger diameter than the small-diameter
portion 175. A small-diameter piston 180 and a large-diameter
piston 182 are slidably received in the respective small-diameter
and large-diameter portions 174, 175, and these two pistons 180,
182 are connected to each other by a connecting rod 184, so that
the pistons 180, 182 are movable as a unit. Thus, the stroke
adjusting piston 174 consists of the small-diameter and
large-diameter pistons 180, 182 and the connecting rod 184. The
small-diameter portion 175 cooperates with the small-diameter
piston 180 to define a variable-volume chamber 188 communicating
with the pressurizing chamber 30. The cylinder housing 172
cooperates with the small-diameter and large-diameter pistons 180,
182 to define a volume control chamber 190 between the two pistons
180, 182. As in the first embodiment of FIG. 1, the volume control
chamber 190 is connected to the accumulator 72 and the master
reservoir 76 through the pressure increase control valve 122 and
the pressure reduction control valve 124, respectively. The large
diameter portion 175 cooperates with the large-diameter piston 182
to define an atmospheric chamber on the side of the piston 182
remote from the volume control chamber 190. The atmospheric chamber
is held in communication with the atmosphere. A spring 192 is
disposed in the atmospheric chamber to bias the stroke adjusting
piston 174 in a direction of reduction of the volume of the
variable volume chamber 188.
While the brake pedal 10 is in the non-operated position, the
stroke adjusting piston 174 is placed in its original or neutral
position in which a force acting on the piston 174 based on the
fluid pressure in the volume control chamber 190 is equal to the
biasing force of the spring 192. As the fluid pressure in the
volume control chamber 190 is increased, the stroke adjusting
piston 174 is moved from the original position in the left
direction as seen in FIG. 13, causing an increase in the volume of
the variable-volume chamber 188, and resulting a flow of the fluid
from the pressurizing chamber 30 into the variable-volume chamber
188. As the fluid pressure in the volume control chamber 190 is
reduced, the piston 174 is moved from the original position in the
right direction, causing a decrease in the volume of the
variable-volume chamber 188, resulting in a flow of the fluid from
the variable-volume chamber 188 into the pressuring chamber 30.
Thus, by controlling the fluid pressure in the volume control
chamber 190, the volume of the variable-volume chamber 1808 is
changed, so that the amount of the fluid in the pressurizing
chamber 30 is accordingly changed to adjust the operating stroke of
the brake pedal 10.
In the present second embodiment wherein the spring 192 biases the
stroke adjusting piston 174 in the direction of reduction of the
volume of the variable-volume chamber 188, as described above, a
reduction of the fluid pressure in the variable-volume chamber 190
will cause the stroke adjusting piston 174 to be moved by the
biasing force in the right direction. In this arrangement, a
reduction of the fluid pressure in the chamber 190 due to an
abnormality of the pump 70, accumulator 72, pressure increase
control valve 122 or pressure reduction control valve 124 would not
cause an increase in the operating stroke of the brake pedal
10.
It is to be understood that the stroke adjusting device 128 and the
solenoid-operated shut-off valve 62 are not essential. The fluid
pressurizing characteristic of the master cylinder 12, that is, the
relationship between the operating force F of the brake pedal 10
and the master cylinder pressure P.sub.M can be controlled as
desired, without the provision of the stroke adjusting device 128.
It is also noted that the fluid pressuring characteristic of the
master cylinder 12 can be controlled, without the provision of the
assisting device 81. It is further noted that the solenoid-operated
shut-off valve 108 between the assisting pressure chamber 100 and
the pressurizing chamber 32 is not essential.
Referring next to FIG. 14, there is shown a hydraulically operated
braking system constructed according to a fourth embodiment of this
invention, wherein a solenoid-operated shut-off valve 210 is
provided between the master reservoir 76 and the assisting pressure
chamber 100, in place of the shut-off valve 108 provided between
the assisting pressure chamber 100 and the pressurizing chamber 32.
This shut-off valve 210 is normally placed in the open position.
When the brake pedal 10 is operated while the assisting device 82
is normal, the shut-off valve 210 is brought to its closed
position, so that the fluid pressure in the assisting pressure
chamber 100 is controlled by controlling the pressure increase and
pressure reduction control valves 74, 75. In the event of
occurrence of the first kind of abnormality of the assisting device
81, the solenoid coil of the shut-off valve 210 is de-energized to
place the shut-off valve 210 in the open position for fluid
communication of the assisting pressure chamber 100 with the master
reservoir 76, so that an operation of the brake pedal 10 causes the
fluid to be supplied from the master reservoir 76 into the
assisting pressure chamber 100, permitting the 10, pressurizing
piston 92 to be moved as the brake pedal 10 is depressed. Thus, the
brake pedal 10 can be operated even in the event of occurrence of
the first kind of abnormality of the assisting device 81. When the
brake pedal 10 is released, the fluid is returned from the
pressurizing pressure chamber 100 back to the master reservoir 76
through the shut-off valve 210. In this event, the assisting drive
force applied to the pressurizing piston 34 is zero, the fluid
pressure generated in the pressurizing chambers 30, 32 is based
solely on the primary drive force based on the operating force F of
the brake pedal 10.
A hydraulically operated braking system according to a fifth
embodiment of the present invention will be described by reference
to FIG. 15, wherein the master cylinder 12 incorporates an
assisting cylinder within a single cylinder housing. This
arrangement has an advantage of a reduced number of parts of the
braking system. Described in detail, the master cylinder 12 has a
pressurizing piston 220 and a piston rod 221 which is fixed to the
piston 220 and connected to the brake pedal 10. The piston 220
cooperates with the cylinder housing to define a pressurizing
chamber 222 on one side of the piston 220 remote from the piston
rod 221, and an assisting pressure chamber 224 on the other side of
the piston 220. The assisting pressure chamber 224 is connected to
the accumulator 72 through the pressure increase control valve 75,
as in the first embodiment of FIG. 1. An increase of the fluid
pressure in the assisting pressure chamber 224 will causes an
increase in the force acting on the pressurizing piston 220.
Reference numeral 225 denotes a stop which determines a fully
retracted position of the pressurizing piston 220.
In the present embodiment of FIG. 15, the pressurizing piston 220
functions also as an assisting piston, and the distance L.sub.M
between the fulcrum 96 of the brake pedal 10 and the rod 221 of the
pressurizing piston 220 is equal to the distance L.sub.S between
the fulcrum 96 and the rod 221 of the assisting piston 220. It will
also be understood that a pressure-receiving surface area S.sub.S
of the assisting piston 220 is equal to the pressure-receiving
surface area S.sub.M of the pressurizing piston 220 minus a
transverse cross sectional area S.sub.O of the piston rod 221. That
is, S.sub.S =S.sub.M -S.sub.O.
Therefore, the master cylinder pressure P.sub.M is expressed by the
following equation:
It is noted that the assisting cylinder 78 may be disposed in
series with the master cylinder 12 such that these cylinders 78, 12
have separate housings.
Referring next to FIG. 17, there is shown a hydraulically operated
braking system according to a sixth embodiment of this invention,
wherein a check valve 230 is provided between the assisting
pressure chamber 100 and the master reservoir 76. The check valve
230 allows a flow of the working fluid in a direction from the
master reservoir 76 towards the assisting pressure chamber 100, and
inhibits a flow of the fluid in the opposite direction.
In the event of occurrence of the first kind of abnormality of the
assisting device 81, the shut-off valve 108 is opened permitting
the fluid communication between the assisting pressure chamber 100
and the pressurizing chamber 32. However, the shut-off valve 108
may be held in its closed state due to an abnormality thereof such
as sticking due to a foreign matter contained in the working fluid.
In this case, the assisting pressure chamber 100 is disconnected
from both of the accumulator 72 and the master reservoir 76, and
the fluid flows into and from the assisting pressure chamber 100
are inhibited. The check valve 230 is provided to prevent this
drawback. The check valve 230 permits the fluid to be supplied from
the master reservoir 76 to the assisting pressure chamber 100,
thereby permitting an operation of the brake pedal 10 even if the
shut-off valve 108 is kept closed due to its abnormality. In this
embodiment, the spring 104 of the pressure reduction control valve
75 has a considerably small biasing force, so that the fluid can be
returned from the assisting pressure chamber 100 back to the master
reservoir 76 through the pressure reduction control valve 75,
without an energization of the coil 102 of the control valve 75,
when the brake pedal 10 is released.
The solenoid-operated shut-off valve 108 may be replaced by a
pilot-operated switch valve which is mechanically switched from its
closed state to its open state, when the fluid pressure in the
accumulator 72 falls below a predetermined lower limit, that is,
falls down to an abnormally low level. Further, a flow restrictor
device may be provided in series with the shut-off valve 108 or the
pilot-operated switch valve.
Referring to FIG. 18, there will be described an example of such a
modification as indicated above, according to a seventh embodiment
of this invention. In this embodiment, a pilot-operated switch
valve 242 and a flow restrictor device 244 as indicated above are
provided in series in a fluid passage 240 connecting the assisting
pressure chamber 100 and the pressurizing chamber 32. The flow
restrictor device 244 includes a check valve 246, a differential
shut-off valve 248 and an orifice 250. The check valve 246 allows a
flow of the fluid in a direction from the assisting pressure
chamber 100 towards the pressurizing chamber 32, and inhibits a
flow of the fluid in the opposite direction. The differential
shut-off valve 248 allows a flow of the fluid in the direction from
the pressurizing chamber 32 towards the assisting pressure chamber
100 when the fluid pressure in the pressurizing chamber 32 becomes
higher than that in the assisting pressure chamber 100 by a
predetermined amount P1. The orifice 250 is disposed in series
connection with the differential shut-off valve 248. The series
connection of the shut-off valve 248 and the orifice 250 is
parallel with the check valve 246.
When the fluid pressure in the accumulator 72 falls below the
predetermined lower limit, the switch valve 242 is brought to the
open state. However, the assisting pressure chamber 100 is
disconnected from the master reservoir 32 by the flow restrictor
device 244 while the fluid pressure difference of these chambers
100, 32 is smaller than the predetermined amount P1. When the brake
pedal 10 is operated in this condition, the fluid is supplied from
the master reservoir 76 into the assisting pressure chamber 100,
thereby permitting the assisting piston 92 to be moved. When the
fluid pressure in the pressurizing chamber 3 has become higher than
the fluid pressure in the assisting pressure chamber 100 by the
predetermined amount P1 or more, as a result of increase of the
brake operating force F, the pressurized fluid is fed from the
pressurizing chamber 32 into the assisting pressure chamber 100
through the differential shut-off valve 248, whereby the braking
operating force F is boosted.
The fluid communication of the assisting pressure chamber 100 with
the pressurizing chamber 32 through the switch valve 242 and the
shut-off valve 248 will cause an increase in the operating stroke S
of the brake pedal 10. However, the pressurizing fluid is not
supplied from the pressurizing chamber 32 into the assisting
pressure chamber 100 immediately after the switch valve 242 has
been opened. Accordingly, the operating stroke is more or less
restricted by this time delay. The master cylinder pressure P.sub.M
changes with the operating stroke S, as indicated by one-dot chain
line in the graph of FIG. 19, when the pressurizing charmer 32 and
the assisting pressure chamber 100 are disconnected from each
other, and as indicated by broken line in the graph of FIG. 19,
when these chaffers 32, 100 are connected to each other. The
present embodiment is adapted such that the master cylinder
pressure P.sub.M, changes with the operating stroke S, along the
one-dot chain line as long as the differential shut-off valve 248
is held in the closed state, and along the broken line after the
shut-off valve 248 has been brought to the open position. In
addition, the provision of the orifice 250 provides a delay for the
braking effect to be provided when the operating speed of the brake
pedal 10 is relatively high. That is, although the shut-off valve
248 is relatively quickly opened when the operating speed of the
brake pedal 10 is relatively high, the rate of flow of the fluid
from the pressurizing chamber 32 towards the assisting pressure
chamber 100 is restricted by the orifice 250.
When the brake pedal 10 is released, the fluid is returned from the
assisting pressure chamber 100 to the pressurizing chamber 32
through the switch valve 242 and the check valve 246, and then to
the master reservoir 76 through the pressurizing chamber 32.
The differential shut-off valve 248 may be a solenoid-operated
shut-off valve whose opening pressure difference is controllable,
like the pressure increase control valve 74. In this case, the
predetermined amount P1 (indicated in the graph of FIG. 19) at
which the curve along which the master cylinder pressure P.sub.M
changes with the operating stroke S is changed from the one-dot
chain line to the broken line can be changed. Further, the switch
valve 242 may be replaced by a mechanically operated or
solenoid-operated switch valve which is brought to its open state
when the fluid pressure in the pressurizing chamber 32 has become
higher than that in the assisting pressure chamber 100 by a
predetermined amount while the fluid pressure in the accumulator 72
is lower than the predetermined lower limit. In this case, the
master cylinder pressure P.sub.M changes with the operating stroke
S along the broken line of FIG. 19 after the chambers 32, 100 has
been brought into communication with each other through the
mechanically operated or solenoid-operated switch valve. It is also
noted that the orifice 250 is not essential. An increase in the
operating stroke S is limited since the shut-off valve 248 is not
opened immediately after the switch valve 242 has been opened.
While the pump 70 and the accumulator 72 are used commonly for both
of the stroke adjusting device 128 and the assisting device 81, two
sets of pump and accumulator may be provided for the two devices
128, 81, respectively. In this case, the operating stroke S when
the chambers 100, 32 are communicated with each other can be
restricted by the stroke adjusting device 128.
At least one of the assisting device 81 and the stroke adjusting
device 128 includes an electric motor for activating these devices
81, 128. For instance, a hydraulically operated braking system
according to an eighth embodiment of this invention shown in FIG.
20 includes two electric motors 262, 268. The electric motor 262 is
connected to an assisting rod 260 through a motion converting
device 264. The assisting rod 260 engages the brake pedal 10. The
electric motor 268 is connected to a volume-changing piston or
stroke adjusting piston 266 through a motion converting device 269.
The motion converting devices 264, 269 are adapted to convert
rotary motions of the electric motors 262, 268 into linear motions
of the assisting rod 260 and stroke adjusting piston 266,
respectively. The electric motors 262, 268 are connected to a motor
control device 270 through respective driver circuits 272, 274, so
that the motors 262, 268 are controlled by the motor control device
270. An assisting electric drive force to be applied to the
assisting rod 260 is controlled by controlling the electric motor
262, and the volume of the variable-volume chamber 116 is
controlled by controlling the electric motor 268. The present
eighth embodiment does not require the pump 70, accumulator 72,
pressure increase control valves 74, 122 and pressure reduction
control valves 75, 124. Accordingly, the space required for
installing the braking system is reduced in the event of an
abnormality of the electric motor 262 or an abnormality associated
with the electric motor 262, the solenoid-operated shut-off valve
62 is closed, so that the braking pressure to be applied to the
wheel brake cylinders 22, 24 can be increased by reducing the
volume of the variable-volume chamber 116. The electric motors 262,
268 may be replaced by electric actuators each including a
piezoelectric element or elements. In this case, the motion
converting devices are not necessary. However, forces generated by
the piezoelectric elements may be applied to the assisting rod 260
and stroke adjusting piston 266 through respective motion
converting devices.
The solenoid-operated shut-off valve 62 may be brought to its
closed state when the master cylinder 12 is not normally operable
to generate the fluid pressure in the pressurizing chamber 30. For
instance, the shut-off valve 62 may be closed when the fluid
pressure detected by the master cylinder pressure sensor 156 is
lower than a predetermined lower limit.
In the embodiments described above, the rods 95, 98 of the
assisting and pressurizing pistons 92, 34 engage the brake pedal 10
such that the engaging ends of the rods 95, 98 are movable relative
to the brake pedal 10 in the longitudinal direction of the brake
pedal 10. However, this arrangement is not essential. That is,
where the rods 95, 98 are connected to the pistons 92, 34 pivotably
relative to the pistons 92, 34, the rods 95, 98 are pivotable
relative to the brake pedal 10 provided the rods 95, 98 engage the
brake pedal 10 pivotably thereto.
Referring next to FIG. 21, there will be described a hydraulically
operated braking system constructed according to a ninth embodiment
of the present invention, wherein the master cylinder and the
assisting cylinder are provided in a single integral housing, in
series connection with each other. This braking system is designed
for a rear-drive vehicle wherein the rear wheels 14, 16 are drive
wheels (driven by a drive power source) while the front wheels 18,
20 are driven wheels.
The braking system of FIG. 21 includes a master cylinder 300 having
two pressurizing chambers 302, 304. The first pressurizing chamber
302 is connected through a flu passage 306 to the wheel brake
cylinders 26, 28 for the front wheels 18, 20, while the second
pressurizing chamber 304 is connected through a fluid passage 308
to the wheel brake cylinders 22, 24 for the rear wheels 14, 16. As
in the embodiments described above, the solenoid-operated shut-off
valves 44 are provided in the fluid passages 306, 308. Further,
normally-open solenoid-operated shut-off valves 312 are provided in
a fluid passage connecting the wheel brake cylinders 22, 24 and the
reservoir 76, while normally-closed solenoid-operated shut-off
valves 316 are provided in a fluid passage 314 connecting the wheel
brake cylinders 26, 28 and the reservoir 76.
These solenoid-operated shut-off valves 312, 316 are closed to
increase the fluid pressures in the wheel brake cylinders 22, 24,
26, 28, and are opened to reduce the fluid pressures in these wheel
brake cylinders. When the brake pedal 10 is released, too, these
shut-off valves 312, 316 are opened to return the fluid from the
wheel brake cylinders 22-28 to the reservoir 76. When the brake
pedal 10 is released, the normally-closed shut-off valves 316 are
kept open for a predetermined time suitable for the fluid to be
able to be completely returned to the reservoir 76, and are then
held in the closed state. The solenoid coils of the shut-off valves
312, 316 are connected through respective driver circuits to both
of the pressure control device 80 and an emergency control device
318. When the assisting device 81 is normally operable, the
shut-off valves 312, 316 are controlled by the pressure control
device 80 in the event of an abnormality of the pressure control
device 80 due to some electrical defect thereof, the shut-off
valves 312, 316 are controlled by the emergency control device 318,
so that the wheel brake cylinders 22-28 can be normally activated
even in the event of an electrical defect associated with the
pressure control device 80.
The master cylinder 300 has a cylinder housing 320 which houses a
first pressurizing piston 322 movable relative to the cylinder
housing 320, and a second pressurizing piston 324 movable to the
first pressurizing piston 322. The first pressurizing piston 322 is
operatively connected to the brake pedal 10, so that the piston 322
is moved in response to an operation of the brake pedal 10. The
second piston 324 divides the interior space of the cylinder
housing 320 into the first and second pressurizing chambers 302,
304. The second pressurizing piston 324 includes two cylindrical
pistons 330, 332 each of which is closed at one of its opposite
ends and is open at the other end. These two cylindrical pistons
330, 332 are disposed such that the outer surfaces of their
bottom-walls 340, 342 are opposed to each other in the axial
direction. The cylindrical piston 330 which is remote from the
first pressurizing piston 322 functions as a partition member
separating the first and second pressurizing chambers 302, 304 from
each other will be referred to as "a front second pressurizing
piston" while the other cylindrical piston 332 will be referred to
as "a rear second pressurizing piston".
The outer circumferential surface of the front second pressurizing
piston 330 fluid-tightly and slidably engages an annular radial
wall 334 formed on the inner circumferential surface of the
cylinder housing 320. The rear second pressurizing piston 332 has
two annular radial walls 336, 337 formed on its outer
circumferential surface such that the annular radial walls 336, 337
are spaced apart from each other in the axial direction of the
piston 332. At these annular radial walls 336, 337, the rear second
pressurizing piston 332 fluid-tightly and slidably engages the
inner circumferential surface of the cylinder housing 320.
The second pressurizing chamber 304 is formed in front of the front
second pressurizing piston 330 with the annular radial wall 334
fluid-tightly engaging the inner circumferential surface of the
cylinder housing 320. A spring 338 is disposed in the second
pressurizing chamber 304, to bias the front second pressurizing
piston 330 in the rear direction toward the rear second
pressurizing piston 332, so that the bottom wall 340 of the front
second pressurizing piston 330 is held in abutting contact with an
annular axial protrusion 343 formed on the bottom wall 342 of the
rear second pressurizing piston 332, whereby the pistons 330, 332
are axially movable as a unit (second pressurizing piston 324). The
fully retracted position of the second pressurizing piston 324 is
determined by abutting contact of a rear open end face 346 of the
rear second pressurizing piston 332 with a rear end face 347 of the
cylinder housing 320. The first pressurizing piston 322
fluid-tightly and slidably engages the inner circumferential
surface of the rear second pressurizing piston 332. The first
pressurizing piston 322 and the rear second pressurizing piston 332
cooperate to define a fluid chamber 348 in front of the first
pressurizing piston 322. The bottom wall 342 of the rear second
pressurizing piston 332 has an orifice 350 for fluid communication
between the fluid chamber 348 and an annular chamber 344 which is
defined by the inner circumferential surface of the cylinder body
320, the outer circumferential surface of the second pressurizing
piston 324, the annular radial walls 334, 336 and the annular axial
protrusion 343. The orifice 350 permits the fluid pressures in the
fluid chamber 348 and the annular chatter 344 to be equal to each
other. The first pressurizing chamber 302 consists of the annular
chamber 344 and the fluid chamber 348.
The volume of the first pressurizing chamber 302 (volume of the
fluid chamber 348) is reduced and the fluid pressure in the first
pressurizing chamber 302 is increased, as the first pressurizing
piston 322 is moved toward the second pressurizing piston 324. The
volume of the second pressurizing chamber 304 is reduced and the
fluid pressure in the second pressurizing chamber 304 is increased,
as the second pressurizing piston 324 is advanced. As the second
pressurizing piston 324 is advanced, the volume of the annular
chamber 344 is also reduced to thereby reduce the volume of the
first pressurizing chamber 302. A spring 352 is disposed in the
fluid chamber 348, to bias the first pressurizing piston 322 in the
rear direction.
The first pressurizing piston 322 cooperates with the cylinder body
320 to define an assisting pressure chamber 360 on the side of the
first pressurizing piston 322 remote from the fluid chamber 348.
The assisting pressure chamber 360 is connected to the assisting
drive force control device 109 through a fluid passage 361. An
assisting drive force based on the fluid pressure in the assisting
pressure chamber 360 acts on the first pressurizing piston 322 in
the forward direction so as to boost the operating force F of the
brake pedal 10. Thus, the first pressurizing piston 322 also
functions as an assisting piston. That is, the first pressurizing
piston 322 has a large-diameter portion 362 slidably engaging the
cylinder body 320, and rear and front axial sections of this
large-diameter portion 362 are considered to be the assisting
piston and the pressurizing piston, respectively.
A normally-open solenoid-operated shut-off valve 363 is provided in
the fluid passage 361. This shut-off valve 363 is placed in its
closed state when the braking system is operated to perform a
traction control of the rear drive wheels 14, 16 (by activating the
wheel brake cylinders 22, 24 to control the drive forces of the
rear drive wheels 14, 16 so as to prevent excessive slipping of
these drive wheels, during starting of the vehicle, for example),
or to perform a vehicle turning stability control (by activating a
selected one or ones of the wheel brake cylinders 22-28 so as to
improve the turning stability of the vehicle during turning). The
shut-off valve 363 is held in its open state when the brake pedal
10 is operated. As described below, a pressurized fluid is supplied
from the second pressurizing chamber 304, while inhibiting this
fluid to be supplied to the assisting pressure chamber 360, during
the traction control or the vehicle turning stability control. A
stop 364 is provided in the assisting pressure chamber 360, to
determine the fully retracted position of the first pressurizing
piston 322.
The first pressurizing chamber 302 and the assisting pressure
chamber 370 are connected to each other through a fluid passage
370. A solenoid-operated shut-off valve 372 and a flow restrictor
device 374 are provided in the fluid passage 370, in series with
each other. The flow restrictor device includes a differential
shut-off valve 376, an orifice 377 and a check valve 378. The
shut-off valve 372 is a normally-open valve which is held open
while the solenoid coil is in a de-energized state. This shut-off
valve 372 is closed when the brake pedal 10 is operated, when the
fluid pressures in the wheel brake cylinders 22, 24 for the drive
wheels 14, 16 are increased to perform the traction control or to
apply a brake to one of the drive wheels 14, 16 during the vehicle
turning stability control. In the event of occurrence of the first
kind of abnormality of the assisting device 81 described above, the
shut-off valve 372 is held in its closed state with the solenoid
coil kept de-energized, even when the brake pedal 10 is operated or
the traction or vehicle turning stability control is commanded to
be effected. When the fluid pressure in the first pressurizing
chamber 302 has become higher than the fluid pressure in the
assisting pressure chamber 360 by a predetermined amount, the
pressurized fluid in the first pressurizing chamber 302 is supplied
to the assisting pressure chamber 360 through the differential
shut-off valve 376, orifice 377 and shut-off valve 372.
Similarly, the second pressurizing chamber 304 and the assisting
pressure chamber 370 are connected to each other through a fluid
passage 380 in which a solenoid-operated shut-off valve 382 and a
flow restrictor device 384 are provided. The shut-off valve 382 is
closed when the brake pedal 10 is operated, but is held in its open
state when the traction control or the vehicle turning stability
control is effected. In the latter case, the shut-off valve 382 is
held in the open state while the above-indicated shut-off valve 372
is held in its closed state, so that the pressurized fluid
controlled by the assisting drive force control device 109 is not
supplied to the first pressurizing chamber 302, but is supplied to
the second pressurizing chamber 304 through the shut-off valve 382
and the check valve 386. In the event of occurrence of the first
kind of abnormality of the assisting device 81, the shut-off valve
382 is returned to the open state, causing the pressurized fluid in
the second pressurizing chamber 304 to be supplied to the assisting
pressure chamber 360 through the differential shut-off valve 380,
orifice 389 and shut-off valve 382.
In the present ninth embodiment of FIG. 21, the operating force F
of the brake pedal 10 is detected by a force sensor 390, and the
fluid pressure in the assisting pressure chamber 360 is detected by
an assisting pressure sensor 392, while the fluid pressure in the
first pressurizing chamber 302 is detected by a master cylinder
pressure sensor 394. The force sensor 390 is a relatively
inexpensive sensor which is capable of detecting the operating
force with a relatively high degree of accuracy when the operating
force is relatively small, but with a relatively low degree of
accuracy when the operating force is relatively large. In view of
this fact, the operating force of the brake petal 10 is estimated
on the basis of the fluid pressures in the assisting pressure
chamber 360 and the first pressurizing chamber 302. In the present
embodiment, the force sensor 390 is adapted to detect as the
operating force F a reaction force F' which is applied from the
first pressurizing piston 322 to the brake pedal 10.
The first pressurizing piston 322 receives the operating force F=F'
detected by the force sensor 390, an assisting drive force F based
on the fluid pressure in the assisting pressure chamber 360, and a
force F.sub.M based on the fluid pressure in the fluid chamber 348
of the first pressurizing chamber 302. These forces F', F.sub.S and
F.sub.M have a relationship represented by the following
equation:
The force F.sub.M is a product of the master cylinder pressure
P.sub.M detected by the master cylinder pressure sensor 394 and a
transverse cross sectional area S.sub.M of the large-diameter
portion 362 of the first pressurizing piston 322, that is, F.sub.M
=P.sub.M.times.S.sub.M On the other hand, the assisting drive force
F.sub.S is a product of the fluid pressure P.sub.S detected by the
assisting pressure sensor 392 and the cross sectional area S.sub.M
minus a cross sectional area S.sub.P of the small-diameter portion
of the piston 322, that is, F.sub.S ={P.sub.S.times.(S.sub.M
-S.sub.P)}. Therefore, the operating force F' can be estimated
according to the following equation:
On the other hand, the reaction force F' and the operating force F
acting on the pedal pad 97 have a relationship represented by the
following equation:
Therefore, the operating force F can be estimated according to the
following equation, where the operating force F acting on the pedal
pad 97 is detected by a force sensor:
As described above with respect to the first embodiment of FIG. 1,
L.sub.M represents the distance between the fulcrum 96 of the brake
pedal 10 and the engaging end of the assisting piston 322, while
L.sub.F represents the distance between the pedal pad 97 and the
fulcrum 96.
The first and second pressurizing chambers 302, 304 are connected
to the reservoir 76 through respective fluid passages 398, 399,
while a fluid passage 400 connected to the suction side of the pump
70 is also connected to the reservoir 76. In the present
embodiment, the interior of the reservoir 76 is divided by
partition members 401a and 401b into three fluid chambers to which
the three fluid passages 398, 399 and 400 are connected,
respectively, so that an abnormality of one of three hydraulic
circuit systems associated with the chambers 302, 304 and pump 70
will not have an influence on the other hydraulic circuit
systems.
In the fluid passage 398, there are provided two check valves 402a,
402b in series with each other. These check valves 402a, 402b
inhibit a flow of the fluid in a direction from the first
pressurizing chamber 302 towards the reservoir 67, but permit a
flow of the fluid in the opposite direction. In the presence of the
check valves 402a, 402b inhibiting the fluid flow from the chamber
302 towards the reservoir 76, the fluid pressure in the chamber 302
can be increased with high stability when the first pressurizing
piston 322 is advanced. Further, the check valves 402a, 402b
permitting the fluid flow from the reservoir 76 into the chamber
302 when the volume of the chamber 302 is increased, the fluid
pressure in the chamber 302 is prevented from being lowered below
the atmospheric pressure, when the first and second pressurizing
pistons 322, 324 are retracted. In addition, the provision of the
fluid passage 398 and check valves 402a, 402b eliminates a need of
providing the pressurizing pistons 322, 324 with primary cups or
inlet valves, and a need of giving the pressurizing pistons 322,
324 relatively large operating strokes for opening and closing such
primary cups or inlet valves, whereby the longitudinal dimension of
the master cylinder 300 can be reduced. Similarly, two check valves
404a, 404b are provided in series with each other in the fluid
passage 399 connected to the second pressurizing chamber 304.
The cylinder housing 320 has two openings 406, 40U which are open
in the respective first and second pressurizing chambers 302, 304
and to which the respective fluid passage 398, 399 are connected.
These openings 406, 407 are always open in the chambers 302, 304
irrespective of the axial position of the second pressurizing
piston 324 relative to the cylinder housing 320. As described
above, the second pressurizing piston 324 consists of the front
second pressurizing piston 330 whose outer circumferential surface
fluid-tightly and slidably engages the annular radial wall 334
formed on the inner circumferential surface of the cylinder housing
320, and the rear second pressurizing piston 332 whose annular
radial walls 336, 337 fluid-tightly and slidably engages the inner
circumferential surface of the cylinder housing 320. The opening
406 is positioned so as to avoid closure of the opening 406 by the
annular radial wall 336.
In the assisting drive force control device 108, the fluid passage
400 and the fluid passage 361 are connected to each other through a
fluid passage 410 which by-passes the pressure increase and
pressure reduction control valves 74, 75. In the fluid passage 410,
there are provided two check valves 412, 413 which permit a fluid
flow in a direction from the reservoir 76 towards the assisting
pressure chamber 360, inhibit a fluid flow in the opposite
direction. The fluid passage 410 permits the fluid to be supplied
from the reservoir 76 to the assisting pressure chamber 360, to
thereby prevent the fluid pressure in the chamber 360 to be lowered
below the atmospheric level, when the brake pedal 10 is operated in
the event of an electrical abnormality that prevents energization
of the solenoid coils of the control valves 74, 75 and causes these
control valves to be held in the closed state.
A solenoid-operated shut-off valve 418 is provided between the
pressure reduction control valve 75 and the reservoir 76. This
shut-off valve 418 is opened when the fluid pressure in the
assisting pressure chamber 360 is reduced, that is, when the fluid
is returned from the chamber 360 to the reservoir 76 through the
pressure reduction control valve 75. In the other condition, the
shut-off valve 418 is held in the closed state. The shut-off valve
418 functions to prevent a discharge flow of the fluid from the
assisting pressure chamber 360 through the control valve 75, and
permits the fluid to be discharged from the assisting pressure
chamber 360 to the reservoir 76 when the control valve 74 is open.
In this respect, the shut-off valve 418 may be considered to be a
pressure reduction responsive shut-off valve which is opened when
the pressure reduction control valve 75 is opened. The shut-off
valve 418 may be positioned between the pressure reduction control
valve 74 and a point of connection 419 of the pressure increase
control valve 74 with the fluid passage 361.
The ROM 132 of the pressure control device 80 stores a traction
control program for performing the traction control of the drive
wheels 14, 16, a vehicle turning detecting program for detecting a
turning of the vehicle, a vehicle turning stability control program
for performing the vehicle turning stability control, and an
operating force estimating program for estimating the operating
force F of the brake pedal 10, in addition to the motor control
program for executing the motor control routine illustrated in the
flow chart of FIG. 8, and the assisting drive force control
program, for controlling the fluid pressure in the assisting
pressure chamber 360. To the input portion 133 of the pressure
control device 80, there are connected the wheel speed sensors 140,
146, the assisting pressure sensor 392, the master cylinder
pressure sensor 394, an accelerator operation sensor 420, and a yaw
rate sensor 422. The accelerator operation sensor 420 detects an
operation of an accelerator pedal of the vehicle, and a turning of
the vehicle is detected on the basis of the yaw rate of the vehicle
detected by the yaw rate sensor 422 and the wheel speeds detected,
by the wheel speed sensors 140-146.
When the brake pedal 10 is operated in the braking system of FIG.
21 constructed as described above, the solenoid-operated shut-off
valves 372, 382 are closed, and the shut-off valves 312 for the
rear drive wheels 14, 16 are closed, while the shut-off valve 363
is held in the open state. As the brake pedal 10 is depressed, the
first pressurizing piston 322 is advanced relative to the rear
second pressurizing piston 332 against the biasing force of the
spring 352. When the operating speed of the brake pedal 10 is not
so high, the fluid in the fluid chamber 348 is fed into the annular
chamber 344 through the orifice 350. As the fluid pressure in the
fluid chamber 348 is increased, the rear and front second
pressurizing pistons 332, 330 are advanced against the biasing
force of the spring 338, so that the volume of the annular chamber
344 is reduced, and the fluid pressure in the first pressurizing
chamber 302 is accordingly increased. The fluid pressurized in the
first pressurizing chamber 302 is supplied to the wheel brake
cylinders 26, 28, while the fluid pressurized in the second
pressurizing chamber 304 is supplied to the wheel brake cylinders
22, 24. The fluid pressure in the assisting pressure chamber 360 is
controlled by controlling the pressure increase and pressure
reduction control valves 74, 75, as described above with respect to
the first embodiment of FIG. 1.
When the brake pedal 10 is released, the shut-off valves 312, 316
are opened to return the pressurized fluid from the wheel brake
cylinders 22-28 to the reservoir 76. The shut-off valves 312 for
the rear wheels 14, 16 are held in the open state, while the
shut-off valves 316 for the front wheels 18, 20 are held in the
open state for a predetermined time and are then restored to the
closed state. In the meantime, the pressurized fluid in the
assisting pressure chamber 360 is returned partly to the reservoir
76 through the normally-open pressure reduction control valve 75,
partly to the first pressurizing chamber 302 through the opened
shut-off valve 372 and the check valve 378, and partly to the
second pressurizing chamber 304 through the opened shut-off valve
382 and the check valve 386. As the volumes of the first and second
pressurizing chambers 302, 304 are increased as a result of the
movement of the brake pedal 10 back to the non-operated position,
the fluid is supplied from the reservoir 76 to the chambers 302,
304 through the fluid passages 398, 399, so that the fluid
pressures in the chambers 302, 304 are prevented from being lowered
below the atmospheric level.
Where the brake pedal 10 is operated at a relatively high speed so
as to reduce the volume of the fluid chamber 348, the fluid
pressure in the fluid chamber 348 is rapidly increased due to a
fluid flow restricting function of the orifice 350, so that a
relatively large fluid pressure difference is generated between the
fluid chamber 348 and the annular chamber 346. Accordingly, the
second pressurizing piston 324 is advanced by this fluid pressure
difference, and the volumes of the first and second pressurizing
chambers 302, 304 are reduced so as to rapidly increase the fluid
pressures in these chambers 302, 304 and the fluid pressures in the
wheel brake cylinders 22-28. This arrangement is effective to
reduce a delay in the braking effect in an initial portion of the
operation of the brake pedal 10 at a relatively high speed.
As the fluid pressure in the fluid chamber 348 is, increased, the
operating force F' as detected by the force sensor 390 is
increased, and the fluid pressure in the assisting pressure chamber
360 is controlled so as to increase with the detected operating
force F', so that the fluid pressure in the fluid chamber 348 is
further increased to increase, the fluid pressure in the wheel
brake cylinders 22-28.
When the account of slip of the drive wheels 2 has become
excessively large with respect to the friction coefficient of the
road surface, that is, when the predetermined condition for
initialing the traction control of the drive wheels 22, 24 is
satisfied, the shut-off valves 382 is held in the open state, and
the shut-off valves 372 and 363 are closed. The pressurized fluid
whose pressure is controlled by the assisting drive force control
device 109 is supplied to the second pressurizing chamber 304, but
is not supplied to the assisting pressure chamber 360 and the first
pressurizing chamber 302. Thus, only the fluid pressure in the
wheel brake cylinders 22, 24 for the rear drive wheels 14, 16 is
increased to brake the drive wheels 14, 16, without an operation of
the brake pedal 10. The fluid pressure in the wheel brake cylinders
22, 24 is controlled by controlling the shut-off valves 44, 312, so
as to hold the slipping amount of the drive wheels 14, 16 within an
optimum range.
In the traction control, the second pressurizing piston 324 is
placed in its fully retracted position, so that the pressurized
fluid supplied to the second pressurizing chamber 304 will not
causes a retracting movement of the second pressurizing piston 324.
Thus, the fluid pressure in the second pressurizing chamber 304 can
be increased while the volume of the first pressurizing chamber 302
is held constant. In other words, only the fluid pressure in the
wheel brake cylinders 22, 24 for the drive wheels 14, 16 can be
increased, without increasing the fluid pressure in the wheel brake
cylinders 26, 28 for the driven wheels 18, 20. Further, the first
pressurizing piston 322 is permitted to be advanced while the
second pressurizing piston 324 is placed in the fully retracted
position. Therefore, the fluid pressure in the first pressurizing
chamber 302 can be increased immediately after an operation of the
brake pedal 10 during the traction control, so that the driven
wheels 18, 20 can also be braked with a high response to the
operation of the brake pedal 10 during the traction control. The
operation of the brake pedal 10 during the traction control causes
the shut-off valve 363 to be opened to supply the pressurized fluid
from the assisting drive force control device 108 to the assisting
pressure chamber 360. An increase in the operating stroke of the
brake pedal 10 operated during the traction control can be
restricted by closing the shut-off valve 382.
If the vehicle, has an excessive spinning or drift-out tendency
during turning, the vehicle turning stability control is initiated
to remove this tendency. The vehicle turning stability control is
effected to generate a difference between the fluid pressures in
the wheel brake cylinders 22, 24 for the right and left drive
wheels 14, 16, so as to give a yaw moment to the vehicle for
thereby eliminating the excessive spinning or drift-out tendency of
the turning, vehicle. In the vehicle turning stability control, the
shut-off valves 372, 373 are closed while the shut-off valve 382 is
held open, as in the traction control. The fluid pressures in the
wheel brake cylinders 22, 23 for the drive wheels 14, 16 are
controlled independently of each other, by controlling the shut-off
valves 44, 312.
When an automatic brake is applied, to the vehicle in an emergency,
the solenoid-operated shut-off valves 363, 372, 382 are opened, and
the pressurized fluid is supplied from the accumulator 72 to the
assisting pressure chamber 360 and the first and second
pressurizing chambers 302, 304.
In the event of occurrence of an electrical abnormality of the
braking system, all of the solenoid-operated shut-off valves are
returned to their original states shown in FIG. 21. When the brake
pedal 10 is operated in this condition, the fluid is supplied from
the reservoir 76 to the assisting pressure chamber 360 through the
check valves 412, 413, to prevent the fluid pressure in the chamber
360 from being lowered below the atmospheric, pressure. When the
fluid pressure in the first pressurizing chamber 302 has become
higher than that in the assisting pressure chamber 360 by the
opening pressure difference of the differential shut-off valve 376
or more, the pressurized fluid in the first pressurizing chamber
302 is supplied to the assisting pressure chamber 360 through the
differential shut-off valve 376, orifice 377 and shut-off valve
372. When the fluid pressure in the second pressurizing chamber 304
nas become higher than that in the assisting pressure chamber 360
by the opening pressure difference of the differential shut-off
valve 388 or more, the pressurized fluid in the second pressurizing
chamber 304 is supplied to the assisting pressure chamber 300
through the differential shut-off valve 388, orifice 389 and
shut-off valve 382. Thus, the fluid pressures in the wheel brake
cylinders 22-28 can be increased while restricting an increase in
the operating stroke of the brake pedal 10. Further, since the
shut-off valve 418 is in the closed state, the pressurized fluid in
the assisting pressure chamber 360 is prevented from being
discharged into the reservoir 76 through the pressure reduction
control valve 74. In addition, the provision of the orifices 377,
389 is effective to reduce a delay of the braking effect when the
operating speed of the brake pedal 10 is relatively high. In the
event of an electric abnormality of the braking system, the
solenoid-operated shut-off valves 312, 316 is controlled by the
emergency control device 318, so that the braking system can be
operated.
In the present ninth embodiment of FIG. 21, the assisting device 81
provides a second pressurizing chamber pressurizing device for
pressurizing the fluid in the second pressurizing chamber 304, and
the rear open end face 346 of the second pressurizing piston 324
and the rear end facet of the cylinder housing 320 cooperate to
provide a volume reduction preventing device for preventing
reduction of the volume of the first pressurizing chamber 302 when
the fluid pressure in the second pressurizing chamber 304 is
increased by the second pressurizing chamber pressurizing device.
Further, the assisting pressure sensor 392, master cylinder
pressure sensor 394 and a portion of the pressure control device 80
assigned to estimate the operating force F' of the brake pedal 10
on the basis of the outputs of these pressure sensors 392, 394
cooperate to provide a brake operating force estimating device for
estimating the operating force F' of the brake pedal 10.
In the hydraulically operated braking system of the ninth
embodiment, the shut-off valves 312 for the rear drive wheels 14,
16 are normally open, while the shut-off valves 316 for the front
driven wheels 18, 20 are normally closed. However, it is possible
that the shut-off valves 312 are normally closed while the shut-off
valves 316 are normally open. In this case, the shut-off valve 363
is not essential, since the fluid pressurized in the first
pressurizing chamber 302 as a result of an advancing movement of
the first pressurizing piston 322 by the pressurized fluid supplied
to the assisting pressure chamber 360 during the traction control
is returned to the reservoir 76 through the normally open shut-off
valves 316, so that the wheel brake cylinders 26, 28 are not
activated during the traction control. The configuration of the
second pressurizing piston 324 is not limited to the details of the
illustrated ninth embodiment. For instance, the front and rear
second pressurizing pistons 330, 332 may be formed integrally with
each other, or the front second pressurizing piston 330 may take
the form of a circular disc. Further, the fully retracted position
of the second pressurizing piston 324 need not be defined by the
rear end of the cylinder housing 320. The first and second
pressurizing pistons 322, 324 may be disposed in series, and may
have the same pressure-receiving surface areas. It is needless to
say that the master cylinder and the assisting cylinder may be
provided as separate units. The stroke adjusting device 128 may be
provided in the present braking system of FIG. 21.
Reference is now made to FIG. 22 showing a hydraulically operated
braking system constructed according to a tenth embodiment of this
invention.
The braking system of FIG. 22 uses a master cylinder 500 including
a cylinder body 502 in which there are fluid-tightly and slidably
received a first and a second pressurizing piston 504, 506. The
cylinder body 502 and the two pressurizing pistons 504, 506 define
a first and a second pressurizing piston 508, 510 in front of the
two pistons 504, 506, respectively. The cylinder body 502 and the
first pressurizing piston 504 cooperate to define an assisting
pressure chamber 512 on the side of the piston 504 remote from the
first pressurizing chamber 508. Thus, the first pressurizing piston
504 also functions as an assisting piston. The cylinder housing 502
is provided with a pair of primary cups 514 and a pair of primary
cups 516, and has a port 520 formed between the primary cups 514.
The second pressurizing chamber 51U is connected to the reservoir
76 through the port 520 and a fluid passage 522.
The second pressurizing piston 506 has a communication hole 524
open in the second pressurizing chamber 510. The communication hole
524 is positioned such that the second pressurizing chamber 510
communicates with the port 520 through the hole 524 when the second
pressurizing piston 506 is placed in the original or fully
retracted position with the brake pedal 10 placed in the
non-operated position. When the second pressurizing piston 506 is
advanced by an operation of the brake pedal 10, the communication
hole 524 is closed by the cylinder housing 502 and is disconnected
from the port 520, so that the fluid pressure in the chamber 510
can be increased when the brake pedal 10 is operated. When the
brake pedal 10 is released, the second pressurizing piston 506 is
returned to the fully retracted position in which the communication
hole 520 communicates with the port 520, to permit the pressurized
fluid to be discharged from the second pressurizing chamber 520
into the reservoir 76 through the fluid passage 522. As the volumes
of the first and second pressurizing chambers 508, 510 are
increased, the fluid is supplied from the reservoir 76 through the
fluid passage 522 into the chambers 508, 510 with elastic
deformation of the primary cups 514, so that the fluid pressures in
the chambers 508, 510 are prevented from being lowered below the
atmospheric level.
The first pressurizing chamber 508 and the assisting pressure
chamber 512 are fluid-tightly separated from each other by the pair
of primary cups 516. The master cylinder 500 of the present
embodiment is not a piston type cylinder, but is a Girling or
plunger type cylinder using the primary cups 514, 516 held by the
cylinder housing 502.
A spring 517 is disposed in the second pressurizing chamber 510, to
bias the second pressuring piston 506 towards the fully retracted
position, while a spring 518 is disposed in the first pressurizing
chamber 508, to bias the first pressuring piston 504 towards the
fully retracted position via retainers 519a, 519b.
The cylinder housing 502 has ports 526, 530 and 534 in addition to
the port 520 indicated above. The port 526 is open to the second
pressurizing chamber 310 and is connected to a fluid passage 528,
which is connected to a fluid passage 536 connected to the port 534
open to the assisting pressure chamber 512. Thus, the second
pressurizing chamber 510 is connected to the assisting pressure
chamber 512 through the port 526, fluid passages 528, 536 and port
534. The port 530 is open to the first pressurizing chamber 508 and
is connected to a fluid passage 531 which is connected to the fluid
passage 528. Therefore, the first and second pressurizing chambers
508, 510 and the assisting pressure chamber 512 are connected to
each other through the fluid passages 528, 530, 536. The fluid
passage 531 is connected also to a fluid passage 532 connected to
the reservoir 76. A check valve 533 is provided in the fluid
passage 532. This check valve 533 allows a flow of the fluid in a
direction from the reservoir 76 towards the first pressurizing
chamber 508, but inhibits a flow of the fluid in the opposite
direction. The assisting pressure chamber 512 is connected to an
assisting drive force control device 538 through the port 534 and
the fluid passage 536.
A normally-open solenoid-operated shut-off valve 542 is provided in
a portion of the fluid passage 528 between the second pressurizing
chamber 510 and a point of connection to the fluid passage 531
connected to the first pressurizing chamber 508. A normally-open
solenoid-operated shut-off valve 546 and a flow restrictor device
547 are provided in a portion of the fluid passage 528 between the
above-indicated point of connection and the fluid passage 536
connected to the assisting pressure chamber 512. Like the flow
restrictor devices 374, 384 in the ninth embodiment of FIG. 21, the
flow restrictor device 547 includes a differential shut-off valve
548 and a check valve 550.
Like the assisting drive force control device 109 provided in the
preceding embodiments, the assisting drive force control device 538
includes the pressure increase control valve 74, pressure reduction
control valve 74, pump 70 and accumulator 72. The flow respirator
device 538 further includes two normally-closed solenoid-operated
shut-off valves 560, 562.
The shut-off valve 560, which functions as an emergency closure
valve is disposed between the assisting pressure chamber 512 and
the solenoid-operated pressure control valve device 82 which
includes the pressure increase and pressure reduction control
valves 74, 75. While the brake pedal 10 is in operation, the
shut-off valve 560 is held in the open state if the
solenoid-operated pressure control valve device 82 is normal, but
is restored to the closed state if the valve device 82 becomes
abnormal. The shut-off valve 560 placed in the closed state
prevents a continuous discharge flow of the pressurized fluid from
the assisting pressure chamber 512 into the reservoir 76 through
the pressure reduction control valve 75, and a continuous supply
flow of the pressurized fluid from the accumulator 72 into the
assisting pressure chamber 512 through the pressure increase
control valve 74. The assisting pressure chamber 512 is connected
to the reservoir 76 through a reservoir passage 564 which by-passes
the solenoid-operated pressure control valve device 82 and the
shut-off valve 560. The check valves 41Z, 412 are provided in the
reservoir passage 564.
The solenoid-operated shut-off valve 562, which functions as an
emergency high-pressure source communicating device, is disposed in
a by-pass passage 570 which connects the assisting pressure chamber
512 and the accumulator 72, while by-passing the solenoid-operated
pressure control valve device 82 and the shut-off valve 560. The
shut-off valve 562 is normally held in the closed state, but is
brought to the open state if at least one of the pressure increase
and pressure reduction control valves 14, 75 cannot be opened while
the electrical system is normal. The shut-off valve 562 placed in
the open state permits the pressurized fluid to be supplied from
the accumulator 72 to the assisting pressure chamber 512.
The shut-off valve 562 may be disposed in a by-pass passage which
connects the delivery side of the pump 70 and the assisting
pressure chamber 512 while by-passing the solenoid-operated
pressure control valve device 82 and the shut-off valve 560. In
this case, the shut-off valve 562 is opened in the event of
occurrence of an abnormality of the pressure control valve device
82 or the shut-off valve 560, so that the fluid delivered from the
pump 70 is supplied to the assisting pressure chamber 512. In this
case, it is desirable to provide the by-pass passage with a check
valve which allows a flow of the fluid in a direction from the pump
70 towards the assisting pressure chamber 512 but inhibits a flow
of the fluid in the opposite direction.
In the hydraulically operated braking system of FIG. 22 constructed
as described above, the control valves 74, 75 and the
solenoid-operated shut-off valves 542, 546, 560, 562 are controlled
as indicated in the table of FIG. 23. In a normal braking operation
with the brake pedal 10 being depressed, the pressure increase and
pressure reduction control valves 74, 75 are controlled to control
the fluid pressure in the assisting pressure chamber 512, in the
same manner as described with respect to the embodiment of FIG. 1,
and the shut-off valve 560 is held in the open state while the
shut-off valve 546 is held in the closed state. When the wheel
braking pressures are increased, the shut-off valve 542 is closed,
and the pressurized fluid from the accumulator 72 is controlled by
the pressure increase control valve 74 and supplied to the
assisting pressure chamber 512 through the shut-off valve 560. When
the wheel braking pressures are reduced, the shut-off valve 542 is
restored to the open state, and the pressurized fluid in the
assisting pressure chamber 512 is returned to the reservoir 76
through the pressure reduction control valve 75, while the
pressurized fluid in the first pressurizing chamber 508 is returned
to the reservoir 76 through the shut-off valve 542 and the second
pressurizing chamber 510. Where the reduction of the wheel braking
pressures is effected with the brake pedal 10 being released, the
pressure reduction control valve 75 is held in the fully open state
for a predetermined time with the maximum electric current being
applied to the solenoid coil. As the volume of the first
pressurizing chamber 508 is increased during releasing of the brake
pedal 10, the fluid is supplied from the reservoir 76 into the
first pressurizing chamber 508 through the fluid passages 532, 531,
so that the fluid pressure in the chamber 508 is prevented from
being lowered below the atmospheric level.
In an automatic braking operation without an operation of the brake
pedal 10, such as a braking operation to effect the vehicle turning
stability control, the pressure increase and pressure reduction
control valves 74, 75 are controlled to control the fluid pressure
in the assisting pressure chamber 512, as in the normal braking
operation. When the wheel braking pressures are increased, the
shut-off valve 542 is closed, and the shut-off valve 546 is opened,
so that the pressurized fluid whose pressure is controlled by the
assisting drive force control device 538 is supplied to not only
the assisting pressure chamber 512 but also the first pressurizing
chamber 508. When the wheel braking pressures are reduced, the
pressurized fluid in the assisting pressure chamber 512 is returned
to the reservoir 76 partly through the pressure reduction control
valve 76, and partly through the shut-off valve 546, check valve
550, shut-off valve 542 and second pressurizing chamber 510. It is
noted that the shut-off valve 542 may be opened when the wheel
braking pressures are increased. In this case, the pressurized
fluid whose pressure is controlled by the assisting drive force
control device 538 is also supplied to the second pressurizing
chamber 510.
In the event of occurrence of an electrical abnormality wherein no
electric current can be applied to the solenoid-operated valves 74,
75, 542, 546, 560, 562, these valves are returned to their original
states indicated in FIG. 22. Upon depression of the brake pedal 10
in this condition, the fluid is supplied from the reservoir 76 to
the assisting pressure chamber 512 through the fluid passage 564
(and the check valves 412, 413), so that the fluid pressure in the
chamber 512 is prevented from being lowered below the atmospheric
pressure. When the fluid pressure in the first and second
pressurizing chambers 508, 510 has become higher than that in the
assisting pressure chamber 512 by the opening pressure difference
of the differential shut-off valve 548 or more, the differential
shut-off valve 548 is opened, so that the pressurized fluid is
supplied from the pressurizing chambers 508, 510 to the assisting
pressure chamber 512, whereby the wheel braking pressures are
increased. In this case, the shut-off valve 560 is returned to the
closed state, to prevent a continuous discharge flow of the fluid
from the assisting pressure chamber 512 through the pressure
reduction control valve 75, and a continuous supply flow of the
fluid from the accumulator 72 into the assisting pressure chamber
512 through the pressure increase control valve 74. Thus, the
shut-off valve 560 in the closed state prevents or minimizes a
variation in the fluid pressure in the chamber 512. When the brake
pedal 10 is released, the fluid is fed from the assisting pressure
chamber 512 to the second pressurizing chamber 510 through the
shut-off valve 546, check valve 550 and shut-off valve 542, and is
returned to the reservoir 76.
Where at least one of the pressure increase control valve 74 and
the shut-off valve 560 cannot be opened and held in the closed
state due to an abnormality while the electric system is normal,
the shut-off valve 562 is opened. For instance, the control valve
74 or the shut-off valve 560 cannot be opened due to sticking of a
valve member due to a foreign matter contained in the fluid. In
this instance in which the pressurized fluid in the accumulator 72
cannot be supplied to the assisting pressure chamber 512 through
the valves 74, 560, the shut-off valve 562 is opened to permit the
pressurized fluid to be supplied from the accumulator 72 to the
chamber 512 through the by-pass passage 570, for increasing the
wheel braking pressures. The abnormality of at least one of the
valves 74, 560 can be detected if the fluid pressure in the
assisting pressure chamber 512 as detected by the assisting
pressure sensor 392 is lower than the predetermined lower limit
even when the valves 74, 560 are commanded to be open. The above
abnormality may also be kilo detected if the actual value of the
fluid pressure in the assisting pressure chamber 512 as detected by
the sensor 392 is lower than the desired or target value by more
than a predetermined value, and if the absolute value of the fluid
pressure difference is not reduced. In the event of occurrence of
the abnormality of the valve 74 and/or the valve 560, the shut-off
valves 542 and 546 as well as the shut-off valve 562 may be
opened.
The shut-off valve 562 may be closed if at least one of the
pressure increase and pressure reduction control valves 74, 75
cannot be closed while the electrical system is normal. This
abnormality of the pressure increase control valve 74 can be
detected if the detected fluid pressure in the assisting pressure
chamber 512 is higher than a predetermined upper limit even when
the pressure increase control valve 74 is commanded to be closed.
The abnormality can also be detected if the detected fluid pressure
in the assisting pressure chamber 512 is higher than the desired or
target value by more than a predetermined value, and if the
absolute value of this pressure difference is increasing.
Similarly, the abnormality of the pressure reduction control valve
75 can be detected. The shut-off valve 562 placed in the closed
state prevents an abrupt variation of the fluid pressure in the
assisting pressure chamber 512 in the event of occurrence of the
abnormality of at least one of the control valves 74, 75.
The shut-off valve 562 and the by-pass passage 570 are not
essential, since an abnormality of the solenoid-operated pressure
control valve device 82 can be dealt with in the same manner as in
the event of an electrical abnormality described above. Further,
the shut-off valve 560 is not essential, since the amount of fluid
pressure variation in the assisting fluid chamber 512 due to the
fluid leakage through the solenoid-operated pressure control valve
device 82 is small where the fluid leakage is not serious. Further,
the fluid passage 532 may be connected to a portion of the fluid
passage 528 between the check valve 550 and the shut-off valve 546,
rather than to the fluid passage 531. In this case, the reservoir
76 and the first pressurizing chamber 508 are connected to each
other through the fluid passages 532, 528 and the two check valves
533, 550.
The cylinder housing 502 may have a port which is formed between
the primary cups 516, to connect the first pressurizing chamber 508
to the reservoir 76 through a fluid passage connected to that port.
In this case, the first pressurizing piston 504 has a communication
hole communicating with the first pressurizing chamber 508 and the
above-indicated port when the piston 504 is in the fully retracted
position. In this arrangement, the fluid is supplied from the
reservoir 76 to the first pressurizing chamber 508 with elastic
deformation of the primary cups 516, and the fluid is returned from
the chamber 508 to the reservoir 76 through the above-indicated
communication hole, port and fluid-passage when the brake pedal 10
is released. Accordingly, it is not necessary to open the shut-off
valve 542 when the brake pedal 10 is released to reduce the wheel
braking pressures.
Referring to FIG. 24, there is illustrated an example of the above
arrangement according to an eleventh embodiment of this invention.
This embodiment uses a master cylinder 600 wherein the cylinder
housing 502 has a port 602 formed between the pair of primary cups
516. The port 602 is connected to a fluid passage 604, which is
connected to the reservoir 76. The first pressurizing piston 504
has a communication hole 606 which is open to the first
pressurizing chamber 508 and which communicates with the port 602
when the piston 504 is in the fully retracted position of FIG. 24.
Accordingly, the fluid is returned from the first pressurizing
chamber 508 to the reservoir 76 through the communication hole 606,
port 602 and fluid passage 604 when the piston 504 is returned to
its fully retracted position. A normally-open solenoid-operated
shut-off valve 607 is provided in the fluid passage 604. This
shut-off valve 607 is closed when the fluid pressure in the
assisting pressure chamber 512 is increased while the brake pedal
10 is in the non-operated position. The first pressurizing chamber
508 is connected to the assisting pressure chamber 512 through a
fluid passage 608 in which a normally-closed solenoid-operated
shut-off valve 610 is disposed. However, a flow restrictor device
is not provided in the fluid passage 608.
The present braking system has an assisting drive force control
device 612 which includes an emergency high-pressure source
communication device 616 as well as the pump 70, solenoid-operated
pressure control valve device 82 and shut-off valves 560. The
emergency high-pressure source communicating device 616 includes a
regulator 614 and a change valve 615.
The regulator 614 is provided in a fluid passage 618 which connects
the accumulator 62 and the assisting pressure chamber 512 while
by-passing the solenoid-operated pressure control valve device 82
and the shut-off valve 560. The regulator 614 is connected to the
change valve 615, reservoir 76 and accumulator 72. The regulator
614 is operated on the basis of the fluid pressure in the first
pressurizing chamber 508, to supply the fluid from the reservoir 76
to the change valve 615, or supply the pressurized fluid from the
accumulator 72 to the change valve 615. The change valve 615 is
connected to the fluid passages 618, 536, and to the port 534
communicating with the assisting pressure chamber 512. The change
valve 615 functions to apply to the assisting pressure chamber 512
the fluid pressure as controlled by the regulator 614 or the fluid
pressure as controlled by the solenoid-operated pressure control
valve device 82 (as applied to the first pressurizing chamber
508).
As shown in FIG. 25, the regulator 614 includes a valve member 620,
a valve seat 622, and a drive member 624. These elements 620, 622,
624 cooperate with the valve housing to define a first fluid
chamber 626, a second fluid chamber 628 and a third fluid chamber
630. The first fluid chamber 626 is connected to the accumulator 72
through a solenoid-operated shut-off valve 632, and the second
fluid chamber 628 is connected to the change valve 615, while the
third fluid chamber 630 is connected to the first pressurizing
chamber 508.
In the state of FIG. 25, the drive member 624 is held in its fully
retracted position under the biasing force of a spring 636, and the
valve member 620 is held seated on the valve seat 722 under the
biasing force of a spring 600, so that the second fluid chamber 628
is disconnected from the first fluid chamber 626 and is
communicated with the reservoir 76 through a fluid passage 640
formed through the valve member 620. In this state, the fluid is
supplied from the reservoir 76 to the change valve 615.
When the fluid pressure in the first pressurizing chamber 508 of
the master cylinder 600 is increased, the drive member 624 is
advanced against the biasing force of the spring 636. When the
fluid pressure in the first pressurizing chamber 508 has been
increased to a level at which the following inequality is
satisfied, the valve member 620 is moved by the drive member 624
away from the valve seat 0.622, and the fluid passage 640 is closed
by the drive member 624:
In the above inequality, Pa, Pc, and P.sub.M represent the fluid
pressures in the first, second and third fluid chambers 626, 628,
630, respectively, that is, the fluid pressure in the accumulator
72, the fluid pressure to be applied to the change valve 615, and
the fluid pressure in the first pressurizing chamber 508 (master
cylinder pressure), respectively, and S.sub.1, S.sub.2 and S.sub.3
represent the cross sectional area of the small-diameter portion of
the valve member 624, the cross sectional area of a communication
passage 642 between the first and second fluid chambers 626, 628,
and the cross sectional area of the large-diameter portion of the
drive member 624. It is noted that the biasing forces of the
springs 636, 638 are ignored in the above inequality.
As a result, the second fluid chamber 628 is disconnected from the
reservoir 76, and is communicated with the first fluid chamber 626,
so that the pressurized fluid of the accumulator 72 is supplied to
the change valve 615.
The regulator 614 is operated such that the fluid pressure to be
applied to the change valve 615 is controlled as represented by the
following equation:
Thus, the fluid pressure in the assisting pressure chamber 512 (the
fluid pressure to be applied to the change valve 615) is controlled
depending upon the fluid pressure in the first pressurizing chamber
508.
The change valve 615 has a first port 650 communicating with the
assisting pressure chamber 512, a second port 652 connected to the
regulator 614, a third port 654 connected to the fluid passage 536
(connected to the solenoid-operated pressure control valve device
82 and the first pressurizing chamber 508), and a spool which is
moved such that the first port 650 is connected to one of the
second and third ports 652, 654 whose fluid pressure is higher, and
is disconnected to the other port 642, 654 whose fluid pressure is
lower. When the fluid pressure at the second port 652 is the same
as that at the third port 654, the spool is placed in a neutral
position in which the first port 650 is connected to both of the
second and third ports 652, 654. When the fluid pressure at the
second port 652 is higher than that at the third port 654, the
fluid pressure controlled by the regulator 614 is applied to the
assisting pressure chamber 512.
The hydraulically operated braking system constructed as described
above is operated as indicated in the table of FIG. 26.
When the brake pedal 10 is in the non-operated position, the
regulator 614 is placed in the original position of FIG. 24, and
the spool of the change valve 615 is placed in its neutral
position. When the brake pedal 10 is depressed, the fluid is
supplied from the reservoir 76 to the assisting pressure chamber
512 through the regulator 614, change valve 615, or alternatively
through the check valves 412, 413 and the change valve 615. Thus,
the fluid pressure in the assisting pressure chamber 512 is
prevented from being lowered below the atmospheric level.
In a normal braking operation, the pressure increase and pressure
reduction control valves 74, 75 are controlled as described above
with the tenth embodiment of FIG. 22. In this case, the shut-off
valve 632 is held in the closed state, so that the pressurized
fluid of the accumulator 72 is not supplied to the regulator 614.
The change valve 615 is held in a state in which the first port
6450 is connected to the third port 654, so that the assisting
pressure chamber 512 is communicated with the solenoid-operated
pressure control valve device 82. Thus, the fluid pressure
controlled by the valve device 82 is applied to the assisting
pressure chamber 512. When the wheel braking pressures are reduced,
the pressurized fluid is returned from the assisting pressure
chamber 512 to the reservoir 76 through the pressure reduction
control valve 75. Since the shut-off valve 607 is placed in the
open state, the pressurized fluid in the first pressurizing chamber
508 is returned to the reservoir 76 through the fluid passage 704,
while the pressurized fluid in the second pressurizing chamber 510
is returned to the reservoir 76 through the fluid passage 522.
In an automatic braking operation, the control valves 74, 75 are
controlled to control the wheel braking pressures in the same
manner as in the normal braking operation. However, the shut-off
valve 610 is opened in the automatic braking operation, the fluid
pressure controlled by the pressure control valve device 82 is
applied to not only the assisting pressure chamber 512 but also the
first pressurizing chamber 508. When the wheel braking pressures
are increased, the shut-off valve 607 is closed, so that the
pressurized fluid in the first pressurizing chamber 508 is
prevented from being returned to the reservoir 76, and the fluid
pressure in the first pressurizing chamber 508 can be increased.
When the wheel braking pressures are reduced, the shut-off valve
607 is returned to the open state, to return the pressurized fluid
from the first pressurizing chamber 508 to the reservoir 76 through
the fluid passage 604.
In the event of an electrical abnormality of the braking system,
all of the solenoid-operated shut-off valves and the
solenoid-operated pressure control valve device 82 are returned to
the original positions of FIG. 24. That is, the shut-off valve 632
is returned to the open state, so that the first fluid chamber 626
of the regulator 614 is communicated with the accumulator 72. When
the fluid pressure in the first pressurizing chamber 508 has been
increased to a level that satisfies the above-indicated inequality,
as a result of an operation of the brake pedal 10, the second fluid
chamber 628 of the regulator 614 is communicated with the first
fluid chamber 626 (accumulator 72). The fluid pressure controlled
by the regulator 614 is applied to the second port 652 of the
change valve 615, and the regulator 615 is operated to connect the
regulator 614 to the assisting pressure chamber 512, so that the
fluid pressure controlled by the regulator 614 is applied to the
assisting pressure chamber 512. Thus, the fluid pressure in the
assisting pressure chamber 512 can be increased to a level
corresponding to the operating force F of the brake pedal 10, by
utilizing the fluid pressure in the accumulator 72, since the
pressure in the accumulator 72 will not be lowered immediately
after the occurrence of the electrical abnormality. When the brake
pedal to is released, the pressurized fluid in the assisting
pressure chamber 512 is returned to the reservoir 76 through the
regulator 614.
In the event of occurrence of an abnormality or defect in the servo
system such as a fluid leakage from the accumulator 72, the
shut-off valves 632, 610 are opened. Since the fluid pressure in
the accumulator 72 is relatively low, the fluid pressure applied to
the change valve 615 through the opened shut-off valve 632 and the
regulator 614 is not so high, even when the fluid pressure in the
first pressurizing chamber 508 is increased by depression of the
brake pedal 10. On the other hand, the fluid pressure in the first
pressurizing chamber 508 is applied to the third port 654 of the
change valve 615 through the open shut-off valve 610, so that the
change valve 615 is operated to connect the first port 650 to the
third port 654. As a result, the fluid pressure in the first
pressurizing chamber 508 is applied to the assisting pressure
chamber 512, whereby the wheel braking pressures can be increased.
When the brake pedal 10 is released, the fluid in the assisting
pressure chamber 512 is returned to the reservoir 76 through the
shut-off valve 610 and the first pressurizing chamber 508.
As described above, the present braking system is arranged such
that the regulator 614 is operated to control the fluid pressure in
the assisting pressure chamber 512 to a level depending upon the
fluid pressure in the first pressurizing chamber 508, even in the
event of occurrence of an electrical abnormality of the braking
system.
It is to be understood that the control of the control valves 74,
75 and shut-off valves 560, 607, 610, 632 is not limited to that of
FIG. 26, which is provided by way of example only. The master
cylinder 600 is not limited to the Girling type, but may be of a
type in which primary cups are provided on the pistons 504, 506. It
is also noted that the shut-off valve 632 is not essential, and may
be eliminated provided the first fluid chamber 626 of the regulator
614 is held in communication with the accumulator 72. In this case,
the change valve 615 is operated to connect the first port 650 to
one of the second and third ports 652, 653 at which the fluid
pressure is higher, so that the higher fluid pressure is applied to
the assisting pressure chamber 512. When the brake pedal 10 is
released, and the fluid pressure in the first pressurizing chamber
508 is reduced, the regulator 614 is operated to connect the change
valve 615 to the reservoir 76, so that the pressurized fluid is
returned from the assisting pressure chamber 512 to the reservoir
76 through the change valve 615 and the regulator 614.
While the several preferred embodiments of the present invention
have been described above in detail, for illustrative purpose only,
it is to be understood that the present invention may be embodied
with various changes, modifications and improvements such as those
described in the SUMMARY OF THE INVENTION, which may occur to those
skilled in the art.
* * * * *